Free RAID Calculator
Optimize your RAID setup by calculating usable capacity, raw capacity, storage efficiency, fault tolerance, parity or mirror overhead, and rebuild-risk notes for common RAID levels including RAID 0, RAID 1, RAID 5, RAID 6, RAID 10, RAID 50, and RAID 60.
Enter your RAID configuration
Choose your RAID level, number of drives, drive size, and unit. For nested RAID levels, you can also set the number of drives per group to estimate usable capacity more accurately.
Raw capacity = Active drives × Drive size
RAID 0 usable = Active drives × Drive size
RAID 1 usable = 1 × Drive size
RAID 5 usable = (Active drives − 1) × Drive size
RAID 6 usable = (Active drives − 2) × Drive size
RAID 10 usable = Active drives ÷ 2 × Drive size
RAID 50/60 usable = Group usable capacity × Number of groups
Free RAID Calculator: Optimize Your RAID Setup for Capacity, Redundancy, and Performance
Building a RAID array is one of those decisions that looks simple on the surface and turns out to be full of consequential trade-offs underneath. You buy a handful of drives, pick a level, and let the controller stripe everything together — except the level you choose silently determines how much of your money becomes usable storage, how many drives can die before your data does, how fast the array reads and writes, and how long you will sweat through a rebuild after a failure. A RAID calculator exists to make those trade-offs visible before you commit hardware to them, and this guide walks through every variable that drives the result.
RAID stands for Redundant Array of Independent Disks, and the core idea has barely changed since the concept was formalized in the late 1980s: combine multiple physical drives into a single logical volume so the group behaves better than any drive could alone — faster, larger, more resilient, or some balance of the three. What changes is the arrangement. RAID 0, 1, 5, 6, 10, 50, and 60 each distribute data differently, and those differences produce dramatically different capacity, fault tolerance, and speed outcomes from the exact same set of disks. Explore the full range of technical planning tools at WalDev, including the engineering calculators category.
This guide covers what RAID actually is and how an array is assembled from caplets of data and parity, why a calculator matters before you spend money on disks, exactly how usable capacity is computed for each level, the fault tolerance each level provides, the read and write performance characteristics, the rebuild-time problem that nobody warns beginners about, a full level-by-level comparison, real-world worked examples, the most common configuration mistakes, and a thorough FAQ. For neighboring engineering math, the Moment of Inertia Calculator and Compression Calculator at WalDev sit alongside this one in the same category.
What is RAID and how does an array actually work?
RAID is a method of combining several physical disks so that the operating system sees one logical volume while the data is physically spread across the underlying drives in a deliberate pattern. The pattern is the whole point. Depending on how the bytes are arranged, the same drives can be tuned for raw speed, for survivability, for maximum capacity, or for a compromise between those goals. The two foundational techniques that every RAID level is built from are striping and mirroring, with a third technique — parity — layered on top to add redundancy without paying the full cost of a full duplicate copy.
Striping means splitting a file into chunks and writing those chunks across multiple drives in parallel. If a 1 GB file is striped across four drives, each drive holds roughly a quarter of it, and all four drives can read or write their portion simultaneously. That parallelism is why striped arrays are fast: aggregate throughput scales with the number of drives because the work is happening on all of them at once. The drawback is that striping alone offers zero protection. Lose any single drive in a pure stripe and the file is incomplete on every drive that survives, which means the file — and usually the entire volume — is gone.
Mirroring is the opposite philosophy: write the same data to two or more drives so that an identical copy always exists. A mirrored pair can lose one drive and keep running without missing a byte, because the survivor holds a complete copy. The cost is efficiency — half of your raw capacity becomes a duplicate rather than usable space. Parity is the clever middle ground. Instead of duplicating data wholesale, a parity block stores a mathematical summary, computed with the exclusive-or (XOR) operation, that lets the array reconstruct any single missing block from the surviving blocks. Parity buys redundancy at the cost of only one drive’s worth of capacity per parity calculation, which is why parity-based levels like RAID 5 and RAID 6 are so popular for bulk storage.
Every named RAID level is simply a recipe that combines these three ingredients in a specific ratio. RAID 0 is pure striping. RAID 1 is pure mirroring. RAID 5 stripes data and distributes a single parity block across the set. RAID 6 adds a second, independent parity calculation. RAID 10, 50, and 60 are nested levels that combine a mirror or parity group with striping across multiple groups. Understanding the ingredients makes the behavior of any level predictable rather than something you have to memorize, and a RAID calculator simply applies the recipe’s math so you can see the result instantly. For a deeper look at how engineering categories organize these tools, visit the engineering calculators category at WalDev.
Striping
Data is split into blocks and written across multiple drives in parallel. This multiplies throughput because every drive works at once, but provides no redundancy on its own — a single drive loss breaks the whole stripe.
Mirroring
An identical copy of the data is kept on a second drive. Survivability is excellent and rebuilds are simple block-for-block copies, but you sacrifice half of your raw capacity to the duplicate.
Parity
A computed XOR summary lets the array rebuild any single lost block from the survivors. It delivers redundancy while consuming only one drive’s worth of capacity per parity calculation — efficient, but slower to write and to rebuild.
A RAID calculator does not replace careful capacity planning or a tested backup strategy — it gives you the numbers to plan with. Knowing in advance that a six-drive RAID 6 array of 8 TB disks yields 32 TB usable rather than 48 TB raw is exactly the kind of insight that prevents a purchasing mistake. Explore more planning tools at WalDev.
Why a RAID calculator matters more than most builders realize
The single most common surprise in storage planning is the gap between the number printed on the box and the number you can actually use. Drives are sold by raw capacity, but a RAID array hands back only a fraction of that, and the fraction depends entirely on the level you select. Someone who buys eight 6 TB drives expecting 48 TB of usable space and then configures RAID 6 will discover they have 36 TB — a 12 TB shortfall that, if the project required 40 TB, just turned a finished build into a second purchasing trip. A calculator surfaces that reality before the money is spent, not after.
The deeper value is in seeing the trade-off curve rather than a single point. The interesting question is rarely “how much space does RAID 5 give me” in isolation — it is “what do I give up in redundancy to gain that capacity, and what do I give up in capacity to gain a second layer of fault tolerance.” When you can adjust drive count and level side by side and watch usable capacity, survivable failures, and relative write performance all move together, the decision stops being a guess and becomes an informed comparison. That is the difference between picking a RAID level because a forum thread recommended it and picking one because the numbers fit your actual requirement.
A calculator is also the fastest way to sanity-check a vendor’s pre-configured appliance or a colleague’s proposed layout. If a NAS vendor advertises a particular usable capacity for a given drive bay count and RAID mode, you can verify the claim in seconds and catch the cases where reserved space, hot spares, or proprietary RAID variants change the math. The same applies when you are sizing the array against a growth forecast: modeling the configuration now, with headroom built in, avoids the painful migration of rebuilding an array onto larger drives a year later. For complementary capacity-and-load planning in the same engineering family, the Rebar Calculator at WalDev applies the same plan-before-you-build discipline to structural material estimation.
Beyond storage planning, WalDev offers a full engineering calculators category covering structural, mechanical, and material calculations — including the Moment of Inertia Calculator, the Stair Stringer Calculator, and the Forge Calculator — all designed for fast, reliable planning.
Understanding every RAID calculator input
A RAID calculator needs only a few inputs, but each one carries assumptions worth understanding before you trust the output. Unlike a financial model with dozens of live market variables, RAID math is deterministic — once you fix the inputs, the capacity and tolerance numbers are exact. The art lies in choosing the inputs that reflect your real situation rather than an idealized one, particularly around drive uniformity and the level’s hidden requirements.
Drive count
The number of physical disks in the array. Every level has a minimum: RAID 0 and RAID 1 need at least two, RAID 5 needs three, RAID 6 needs four, and RAID 10 needs four arranged in even-numbered mirrored pairs. Adding drives increases capacity and, for striped levels, performance — but it also increases the statistical chance that at least one drive fails in a given period, which matters most for single-fault-tolerant levels.
Drive capacity
The size of each individual disk, usually entered in gigabytes or terabytes. The calculator multiplies this by the effective drive count according to the level’s formula. The critical caveat is that almost all RAID levels equalize to the smallest drive in the set, so a mixed-size array wastes the extra space on its larger members. Enter the smallest drive’s capacity for an honest result.
RAID level
The arrangement that determines the recipe — striping, mirroring, parity, or a nested combination. This is the input that changes everything: the same drives produce wildly different usable capacity and fault tolerance depending on the level. Selecting the level is really selecting your position on the capacity-versus-redundancy-versus-performance triangle.
Hot spares (optional)
A hot spare is a drive that sits idle in the array, ready to be automatically pulled into service the moment another drive fails, shortening the window of vulnerability. A hot spare is not part of the usable capacity calculation — it is reserved overhead — so a calculator that accounts for spares will subtract them from the active drive count before applying the level’s formula.
Decimal versus binary capacity. Drive manufacturers advertise capacity in decimal units, where 1 TB equals 1012 bytes. Most operating systems report in binary units, where 1 TiB equals 240 bytes. The result is that a “1 TB” drive shows up as roughly 0.91 TiB, a difference of about 9% that grows with array size. A RAID calculator reports the array-level figure; the formatted, OS-reported number will be slightly smaller still once filesystem overhead is included.
How to use a RAID calculator step-by-step
The workflow below turns the abstract trade-offs into a concrete configuration you can buy and build with confidence. Each step builds on the previous one, and the final step — testing alternatives — is the one that separates a workable array from an optimal one.
Start with the number of disks your chassis or NAS can hold and the capacity of each. If you intend to mix drive sizes, enter the capacity of the smallest one, because most RAID levels will treat every member as that size and strand the rest. Identical drives are strongly preferred for both efficiency and predictable behavior.
Decide what you are optimizing for. Choose RAID 0 only when capacity and speed matter and the data is disposable or backed up elsewhere. Choose RAID 1 or RAID 10 when fast rebuilds and performance matter. Choose RAID 5 or RAID 6 when you want efficient bulk capacity with redundancy, with RAID 6 preferred as drive sizes grow.
The calculator returns usable capacity — raw total minus redundancy overhead. Compare it against your real requirement plus a growth buffer. A common rule of thumb is to plan for the array to be no more than 70–80% full at deployment so you have room to grow and so the filesystem keeps performing well.
Check how many simultaneous drive failures the configuration survives, and think about the rebuild window. With large modern drives, a parity rebuild can take many hours to days, during which the array runs degraded and a second failure could be catastrophic. If the rebuild window worries you, move from RAID 5 to RAID 6 or add a hot spare.
Change the drive count and level and watch how the three outputs move together. Sometimes adding one more drive lets you step up to a more resilient level while still meeting your capacity target. This is where the calculator earns its keep — it lets you explore the whole option space in minutes rather than discovering the trade-off after the hardware is in the rack.
The math behind usable RAID capacity
The capacity formulas are the most concrete part of RAID planning, and they are simpler than the jargon suggests. In every formula below, N is the number of drives and S is the capacity of a single drive (using the smallest drive’s size for mixed arrays). The output is usable capacity before filesystem and binary-conversion overhead.
RAID 0 usable = N × S
RAID 1 usable = S (two-drive mirror; the set's capacity equals one drive)
RAID 5 usable = (N − 1) × S
RAID 6 usable = (N − 2) × S
RAID 10 usable = (N ÷ 2) × S
RAID 50 usable = (N − number_of_parity_groups) × S
RAID 60 usable = (N − 2 × number_of_parity_groups) × S
The logic behind each is direct. RAID 0 keeps everything because it stores no redundancy. RAID 1 keeps the capacity of a single drive because the second drive is a copy. RAID 5 surrenders exactly one drive’s worth of capacity to distributed parity, so usable space is the equivalent of all drives but one. RAID 6 surrenders two drives’ worth because it maintains two independent parity calculations. RAID 10 keeps half because every drive is mirrored. The nested levels RAID 50 and RAID 60 subtract one or two drives per parity sub-group respectively, since each sub-group carries its own parity.
A worked example makes the efficiency differences vivid. Take eight 6 TB drives, a raw total of 48 TB. Under RAID 0 you would have all 48 TB usable but no protection at all. Under RAID 5 you would have (8 − 1) × 6 = 42 TB usable and survive one failure. Under RAID 6 you would have (8 − 2) × 6 = 36 TB usable and survive two failures. Under RAID 10 you would have (8 ÷ 2) × 6 = 24 TB usable, survive at least one failure with a fast rebuild, and gain strong performance. The same eight drives span from 48 TB to 24 TB depending purely on the resilience you want — that 24 TB swing is the cost of redundancy made visible.
Parity efficiency improves as you add drives. RAID 5 across four drives wastes 25% of raw capacity; across eight drives it wastes only 12.5%. That is why large parity arrays are capacity-efficient — but bigger parity sets also mean longer, riskier rebuilds, which is the counterweight discussed in the rebuild section below.
Fault tolerance across RAID levels
Fault tolerance answers a single blunt question: how many drives can fail before your data is lost. It is the redundancy half of the capacity-versus-redundancy trade, and it is where the temptation to maximize usable space runs into the reality of hardware that does eventually fail. The right way to think about tolerance is not just the headline number of survivable failures, but also which specific failures are survivable and how exposed the array becomes during recovery.
RAID 0 tolerates zero failures — any single drive loss destroys the volume, which is why it is reserved for scratch data, caches, or workloads that are reconstructed from another source. RAID 1 and RAID 5 each tolerate exactly one failure: a mirror can lose one half, and a single-parity set can lose one member and rebuild it from the survivors. RAID 6 tolerates two simultaneous failures thanks to its dual parity, which is its entire reason for existing. RAID 10 has a more nuanced answer: it can survive multiple drive failures, but only if no two failures hit the same mirrored pair. In an eight-drive RAID 10 you could lose up to four drives and survive if each loss is from a different pair — or you could lose just two and be wiped out if both belonged to the same mirror.
The nested parity levels inherit their sub-group tolerance. RAID 50 is a stripe across multiple RAID 5 groups, so it survives one failure per group; a configuration of two RAID 5 groups can survive two failures total, provided they land in different groups. RAID 60 is a stripe across multiple RAID 6 groups, surviving two failures per group. This per-group structure is what makes nested levels attractive for very large arrays: it splits a giant set into smaller parity domains, which limits both the rebuild scope and the blast radius of a failure.
| RAID Level | Minimum Drives | Survivable Failures | Redundancy Method |
|---|---|---|---|
| RAID 0 | 2 | 0 | None (pure striping) |
| RAID 1 | 2 | 1 per mirror | Mirroring |
| RAID 5 | 3 | 1 | Single distributed parity |
| RAID 6 | 4 | 2 | Dual distributed parity |
| RAID 10 | 4 | 1+ (one per mirror pair) | Mirroring + striping |
| RAID 50 | 6 | 1 per RAID 5 group | Parity + striping |
| RAID 60 | 8 | 2 per RAID 6 group | Dual parity + striping |
Tolerance is not safety. Even a two-fault-tolerant array can be destroyed by a controller failure, a power surge, ransomware, accidental deletion, or a third concurrent drive failure. Fault tolerance improves availability — it keeps you running through a drive loss — but it is not a substitute for an independent, tested backup. Every serious storage plan pairs RAID with a backup, ideally following a 3-2-1 strategy.
Read and write performance across RAID levels
Performance is the third corner of the triangle, and it behaves differently for reads than for writes. The general principle is that striping helps both reads and writes by spreading work across drives, mirroring helps reads (because either copy can serve a request) but not writes (because both copies must be updated), and parity hurts writes because every write requires reading and recomputing the parity block — the so-called write penalty.
RAID 0 is the performance king for both reads and writes because every drive contributes in parallel and there is no redundancy overhead to pay. RAID 1 offers strong read performance, since a read can be served from whichever mirror is least busy, but write performance is roughly that of a single drive because every write lands on both members. RAID 10 combines the best of both: it stripes across mirrored pairs, so reads scale with all the drives and writes scale with the number of pairs, while still delivering single-drive-class rebuild simplicity. This is why RAID 10 is the default for databases and other write-intensive, latency-sensitive workloads.
Parity levels carry a write penalty that is important to understand. A single small write to a RAID 5 array requires four physical operations: read the old data, read the old parity, write the new data, and write the new parity. That four-operation cost — often described as a write penalty of four — is why RAID 5 underperforms on random-write workloads despite its excellent read throughput. RAID 6 is worse still, with a write penalty of six because it maintains two parity blocks. For large sequential writes the penalty is mitigated because the controller can compute parity for a full stripe at once, which is why parity arrays do fine for media storage, backups, and archival data while struggling with transactional databases.
| RAID Level | Read Performance | Write Performance | Write Penalty |
|---|---|---|---|
| RAID 0 | Excellent (scales with N) | Excellent (scales with N) | 1 |
| RAID 1 | Good (dual-source reads) | Single-drive class | 2 |
| RAID 5 | Very good | Moderate (parity overhead) | 4 |
| RAID 6 | Very good | Lower (dual parity) | 6 |
| RAID 10 | Excellent | Very good | 2 |
| RAID 50 | Very good | Better than RAID 5 | 4 |
| RAID 60 | Very good | Better than RAID 6 | 6 |
The practical takeaway is that there is no universally fastest level — there is only the level whose performance profile matches your workload. A video editor moving large sequential files cares about throughput and may be perfectly happy on RAID 5 or RAID 6. A database administrator handling thousands of small random writes per second will choose RAID 10 every time and accept the capacity cost. For mechanical-systems performance estimation in a similar spirit, the Compression Calculator at WalDev applies analogous trade-off thinking to engine and gas-law calculations.
The rebuild problem and why it matters more every year
The rebuild is the most underappreciated risk in RAID planning, and it has grown more dangerous as drive capacities have ballooned. When a drive fails in a redundant array, the array enters a degraded state and must reconstruct the lost drive’s data onto a replacement. For mirrored levels this is a simple copy. For parity levels it is a full read of every surviving drive combined with parity recomputation — a process that can take many hours on a healthy system and far longer if the array is also serving live traffic.
The danger is twofold. First, throughout the rebuild the array is running with reduced or zero remaining redundancy. A single-parity RAID 5 array that has lost one drive has no protection at all until the rebuild finishes — a second failure during that window means total data loss. Second, the rebuild itself stresses every remaining drive by reading them in full, and drives bought together tend to age and fail together, so the rebuild is precisely the moment a marginal sibling drive is most likely to give out. This is the scenario that prompted the industry’s shift toward RAID 6 and RAID 10 for large arrays.
Drive size is the variable that turned this from a theoretical concern into a practical one. Rebuilding a 2 TB drive might take a few hours; rebuilding a 16 TB or 20 TB drive can take a day or more, multiplying the exposure window roughly tenfold. There is also a subtler statistical concern: every drive has a published unrecoverable read error rate, and reading an entire large array during a rebuild involves so many bytes that the probability of hitting an unrecoverable error somewhere in the set becomes non-trivial. For these reasons, most storage engineers now treat RAID 5 as unsuitable for arrays built from very large drives and default to RAID 6 or RAID 10, where a second drive’s worth of protection covers the rebuild window.
Prefer dual-fault tolerance on large drives. When individual drives exceed roughly 4–8 TB, choose RAID 6 or RAID 10 so the array survives a second failure during the long rebuild window.
Use a hot spare to shrink the vulnerable window. A hot spare lets the rebuild begin automatically the instant a drive fails, rather than waiting for a human to notice and swap hardware.
Limit parity-group size. Smaller parity domains — achieved with nested RAID 50 or 60 — keep each rebuild scoped to fewer drives, reducing both the time and the stress of recovery.
Mix drive batches deliberately. Drives from the same manufacturing batch can fail at similar ages. Sourcing drives from different batches or vendors lowers the chance of correlated failures during a rebuild.
RAID level-by-level comparison
With the underlying mechanics established, here is how each common level behaves in practice and where it earns its place. Think of this as the field guide you consult once the calculator has shown you the numbers and you need to interpret what they mean for your situation.
RAID 0 — Maximum speed and capacity, zero safety
Pure striping with no redundancy. Delivers full raw capacity and the best read and write performance, but a single drive failure destroys everything. Appropriate only for disposable scratch data, render caches, or content that is fully backed up elsewhere and can be rebuilt quickly.
RAID 1 — Simple, reliable mirroring
A straightforward duplicate across two drives. Excellent read performance, trivial and fast rebuilds, and survives one failure — at the cost of half the raw capacity. Ideal for boot volumes, small critical datasets, and anywhere simplicity and recoverability outweigh capacity efficiency.
RAID 5 — Efficient single-parity bulk storage
Striping with one distributed parity drive. Good capacity efficiency that improves with more drives, very good reads, a moderate write penalty, and tolerance for one failure. Best for read-heavy bulk storage on modest drive sizes; increasingly discouraged for large modern drives due to rebuild risk.
RAID 6 — Dual-parity resilience
Striping with two independent parity calculations, surviving two simultaneous failures. The standard choice for large-drive bulk arrays because it protects through the long rebuild window. Costs two drives of capacity and carries a heavier write penalty, but the added safety is usually worth it.
RAID 10 — Performance with fast recovery
Striping across mirrored pairs. Excellent read and write performance, the fastest and simplest rebuilds, and resilience to multiple failures provided no pair loses both members. Costs half the capacity. The go-to for databases, virtualization, and any latency-sensitive write-heavy workload.
RAID 50 / 60 — Nested levels for very large arrays
Stripes across multiple RAID 5 or RAID 6 groups. They scale capacity and performance while keeping parity domains small, which limits rebuild scope and blast radius. Reserved for large-drive-count enterprise arrays where a single flat parity set would be impractically risky to rebuild.
Real-world RAID configuration examples
Abstract formulas become intuitive once you see them applied to concrete situations. The three scenarios below cover the most common real builds — a home media server, a small-business NAS, and a database server — and show how the same calculator logic leads to different optimal answers depending on the priority.
Home media server
Goal: maximum capacity for movies and backups, with protection against a single failure. Six 8 TB drives in RAID 5 yield (6 − 1) × 8 = 40 TB usable and survive one failure. Because the data is large and sequential, the write penalty barely matters, making this a capacity-efficient choice for a read-dominated workload.
Small-business NAS
Goal: resilient shared storage where a long rebuild must not expose the business to data loss. Eight 6 TB drives in RAID 6 yield (8 − 2) × 6 = 36 TB usable and survive two simultaneous failures. The dual parity covers the rebuild window — the right call when uptime and safety outweigh squeezing out every last terabyte.
Database / virtualization host
Goal: low-latency, write-heavy performance with fast recovery. Eight 4 TB drives in RAID 10 yield (8 ÷ 2) × 4 = 16 TB usable. Capacity is sacrificed, but random-write performance and rebuild speed are excellent, which is exactly what transactional and hypervisor workloads demand.
Notice how the same eight-drive chassis lands on three different levels purely because the priority changes. The media server optimizes for capacity, the business NAS for redundancy during recovery, and the database host for write performance and fast rebuilds. None of these is the “best” RAID level in the abstract — each is the best level for its workload, which is the entire point of running the numbers before you build. For another planning discipline where the right answer depends on the load profile, see the Moment of Inertia Calculator at WalDev.
The concept of RAID was formalized in the 1988 paper A Case for Redundant Arrays of Inexpensive Disks (RAID) by Patterson, Gibson, and Katz. The standard RAID levels reference documents the levels and their behavior in detail.
The WalDev engineering calculators category hosts this RAID calculator alongside structural, mechanical, and materials tools for fast technical planning.
Choosing the right RAID level for your needs
Selecting a level is fundamentally about ranking three priorities — capacity, redundancy, and performance — for your specific use case, then picking the level that delivers the top priority without unacceptably sacrificing the others. There is no configuration that maximizes all three at once; the whole exercise is choosing which corner of the triangle to favor and how much of the other two you can give up.
If raw capacity and speed are everything and the data is replaceable, RAID 0 wins, but accept that any failure is total. If recoverability and simplicity matter most for a small, critical dataset, RAID 1 is hard to beat. If you want efficient bulk storage with reasonable protection and your drives are modest in size, RAID 5 is sensible — but the moment your drives get large, step up to RAID 6 so the rebuild window does not become a liability. If you run write-heavy, latency-sensitive workloads and can afford to give up half your capacity, RAID 10 is the performance and recovery champion. And if you are building a very large array with many drives, the nested RAID 50 or 60 levels keep parity domains and rebuild scope manageable.
A useful final filter is to imagine the worst day: a drive has just failed and the array is rebuilding. Ask how comfortable you are running through that window, how long it will take, and what a second failure would cost you. If that thought experiment makes you uneasy with your tentative choice, move up a level of protection. The capacity you give up is cheap insurance against the scenario that actually destroys data. Then pair whatever you choose with a real backup, because no RAID level protects against the failures that live outside the array.
The engineering calculators category at WalDev brings together the RAID Calculator, the Rebar Calculator, the Forge Calculator, and more — a single place to run the numbers before you commit to a build.
Common RAID mistakes to avoid
Most RAID failures in the real world are not exotic — they are repeatable mistakes rooted in misunderstanding what the array does and does not protect against. Knowing them in advance is the cheapest way to avoid them.
This is the most expensive misconception in storage. RAID protects against drive hardware failure and nothing else. It does not protect against deletion, corruption, ransomware, controller failure, theft, or disaster. An array with no separate backup is one accidental delete or one malware infection away from total loss. Always pair RAID with an independent, tested backup.
On large modern drives, a RAID 5 rebuild takes so long and stresses the surviving drives so heavily that the probability of a second failure — or an unrecoverable read error — before the rebuild completes becomes uncomfortably high. For drives of several terabytes and up, RAID 6 or RAID 10 is the safer default.
Because most levels equalize to the smallest drive, dropping a single larger drive into an array of smaller ones wastes the difference. Plan around uniform drives, and if you must mix, calculate using the smallest drive’s capacity so your expectations are honest from the start.
Without a hot spare, the rebuild cannot begin until a human notices the failure and physically swaps a drive — which can be hours or days, all spent in a degraded, vulnerable state. A hot spare lets recovery start automatically and dramatically shortens the exposure window for important arrays.
A redundant array that silently loses a drive offers no protection against the next failure, because nobody knew the first one happened. Drives fail quietly, and an unmonitored array can sit degraded for weeks. Configure email or dashboard alerts so a failure triggers immediate action rather than a delayed discovery during the second failure.
The calculator’s usable figure is the array-level number. The OS will report somewhat less after binary-unit conversion and filesystem overhead. Build that gap into your capacity target, and aim to deploy with meaningful free space rather than filling the array to the brim, where many filesystems lose performance.
Frequently asked questions about RAID calculators and array planning
How is usable RAID capacity calculated?
Usable capacity is the raw total of all drives minus the overhead consumed by redundancy. RAID 0 keeps all of it because it stores nothing extra. RAID 1 and RAID 10 keep half because data is mirrored. RAID 5 loses exactly one drive’s worth of capacity to distributed parity, giving (N − 1) × S. RAID 6 loses two drives’ worth to dual parity, giving (N − 2) × S. The figure a calculator reports is before binary-unit conversion and filesystem overhead, so the operating system will display slightly less.
How many drive failures can a RAID array survive?
It depends on the level. RAID 0 survives none — any failure is total. RAID 1 and RAID 5 survive a single failure. RAID 6 survives two simultaneous failures. RAID 10 can survive several failures as long as no two of them strike the same mirrored pair, so the survivable number depends on which drives fail, not just how many. RAID 50 survives one failure per RAID 5 sub-group, and RAID 60 survives two per RAID 6 sub-group.
What is the difference between RAID 5 and RAID 6?
RAID 5 keeps a single distributed parity block and can rebuild from one drive failure, sacrificing the capacity of one drive. RAID 6 maintains two independent parity calculations and can survive two simultaneous failures, sacrificing two drives’ worth of capacity. RAID 6 is the preferred choice for arrays built from large drives, because the rebuild after a failure takes long enough that a second failure during the window is a realistic risk — and only RAID 6 protects against it.
Why does RAID 10 rebuild faster than RAID 5 or RAID 6?
When a drive fails in RAID 10, recovery is a simple block-for-block copy from the surviving mirror partner — fast and low-stress. Parity levels must instead read every other drive in the set and recompute the missing data through parity math, which is far slower and exercises all remaining drives heavily during the rebuild. That extra stress on already-aged sibling drives is exactly what raises the chance of a second failure mid-rebuild on parity arrays.
Does RAID replace the need for backups?
No, and treating it as a backup is the most dangerous RAID mistake. RAID protects only against drive hardware failure within the array’s tolerance. It offers nothing against accidental deletion, file corruption, ransomware, controller failure, power events, fire, theft, or a failure count that exceeds the array’s tolerance. RAID improves availability and uptime; it does not make data safe. A separate, independent, and regularly tested backup is essential no matter which RAID level you run.
What happens if I mix different drive sizes in an array?
Most RAID levels treat every drive as if it were the size of the smallest member, so the extra space on larger drives is stranded and wasted. A RAID 5 set of one 4 TB drive and three 2 TB drives behaves as four 2 TB drives, leaving 2 TB on the larger drive entirely unused. For efficiency use identical drives; if you must mix, plan and calculate around the smallest drive’s capacity so your expectations match reality. Some software RAID implementations like ZFS or Btrfs handle mixed sizes more gracefully, but the safe planning assumption is the smallest-drive rule.
Why is my reported usable capacity smaller than the calculator shows?
Two effects shrink the real-world number. First, manufacturers advertise capacity in decimal terabytes (1012 bytes) while operating systems usually report binary tebibytes (240 bytes), about a 9% difference that grows with array size. Second, the filesystem, RAID metadata, and reserved space consume additional capacity once the volume is formatted. A RAID calculator reports the array-level usable figure before these formatting overheads, so the OS-reported number will always be somewhat lower.
Is software RAID as good as hardware RAID?
For most users, yes. Modern software RAID — mdadm on Linux, Storage Spaces on Windows, and ZFS or Btrfs — is mature, reliable, and often more flexible than hardware RAID, with the major advantage that the array is not bound to a specific controller card that could become a single point of failure or an obsolescence trap. Hardware RAID still has a place where dedicated parity computation and battery-backed write caching benefit heavy write workloads, but it is no longer a default requirement for a dependable array.
How long does a RAID rebuild take?
It depends on drive size, array activity, and RAID level. A mirror rebuild is a straight copy and finishes relatively quickly. A parity rebuild must read the entire surviving set and recompute data, so it scales with the total capacity involved — a small drive may rebuild in a few hours, while a large multi-terabyte drive can take a day or more, especially if the array continues serving traffic during recovery. Because the array runs degraded throughout, minimizing rebuild time through dual-fault tolerance, hot spares, and smaller parity groups is a core safety strategy.
Where can I find more engineering planning tools?
WalDev hosts a full engineering calculators category covering structural, mechanical, and materials calculations. Related tools include the Rebar Calculator for reinforcement estimation, the Moment of Inertia Calculator for structural analysis, the Stair Stringer Calculator for stair layout, and the Compression Calculator for gas and engine calculations.
Final thoughts on optimizing your RAID setup
A RAID calculator turns an intimidating-looking decision into a transparent comparison of trade-offs. Once you understand the three ingredients — striping, mirroring, and parity — and the three priorities they balance — capacity, redundancy, and performance — every named level becomes predictable rather than mysterious. The calculator’s job is simply to apply the recipe’s math so you can see, instantly, how a given drive count and level convert raw capacity into usable space, how many failures you can survive, and roughly how the array will perform. That visibility is what lets you buy the right hardware once instead of discovering a shortfall after the rack is full.
The habits that separate a resilient storage setup from a fragile one are not complicated. Plan capacity with binary-unit and filesystem overhead in mind and deploy with headroom. Favor dual-fault tolerance once your drives get large, because the rebuild window is the moment of greatest danger. Use a hot spare and active monitoring so recovery starts fast and failures never go unnoticed. Match the level to your workload rather than chasing a single “best” answer. And above all, remember that RAID is about availability, never about safety — pair it with an independent, tested backup every single time.
For authoritative background on how the standard RAID levels are defined and behave, the standard RAID levels reference is a thorough public source rooted in the original 1988 RAID research. And for the full suite of technical planning tools that complement this one, visit WalDev and explore the complete engineering calculators category.
