A 40 pound bag of pool salt costs about the same as a takeaway pizza and contains, by weight, roughly 99.8 percent of exactly the same compound you keep in a grinder on the kitchen table. Which raises an obvious question: what is the other 0.2 percent, why does it matter enough to have its own product, and why do salt system manufacturers care so much about it that using the wrong bag can void a warranty? The answer is a short lesson in why purity specifications exist, and it will save you from the two most expensive mistakes a salt pool owner can make with a cheap substitute.
Pool salt is sodium chloride, refined to a high purity, with nothing added. That is the entire product. It is not a chemical in the sense that chlorine or algaecide is a chemical; it is a mineral, mined or evaporated, cleaned up and bagged. What makes it worth buying specifically rather than grabbing whatever salt is cheapest is what has been taken out rather than anything that has been put in.
If you are here because you are about to buy your first load, the pool salt calculator will tell you how many bags to get based on your pool volume and current reading. If you are here because you are wondering whether the softener pellets in the garage will do, read the purity section carefully before you open them.
What this guide covers
The short answer
Pool salt is granular sodium chloride at 99 percent purity or better, with no iodine, no anti-caking agents and no free-flowing additives. A salt chlorine generator dissolves it in the pool water and splits it electrically to produce chlorine, and because the chlorine reverts to salt after use, the same salt gets recycled almost indefinitely.
Everything distinctive about the product is subtractive. It is not that pool salt has something special in it. It is that the things which cause staining, cloudiness, foaming and scaling have been removed.
The one specification that matters: 99 percent minimum sodium chloride, no additives. Check your generator manual, because several manufacturers specify 99.8 percent and treat anything less as grounds to decline a warranty claim on a stained cell.
What is actually inside the bag
| Component | Typical proportion | Why it is there, or why it matters |
|---|---|---|
| Sodium chloride (NaCl) | 99.0–99.9% | The entire point. Raw material for chlorine generation |
| Moisture | 0.05–0.3% | Unavoidable; why salt cakes if stored damp |
| Calcium and magnesium | Trace | Contribute to calcium hardness and cell scaling if excessive |
| Sulphate | Trace | Harmless at these levels |
| Insolubles (sand, clay) | Under 0.1% in good salt | Settle on the pool floor; higher in cheap rock salt |
| Iron and manganese | Should be near zero | The staining culprits. Even tiny amounts show up over time |
| Iodine | None | Present in table salt; can discolour water |
| Anti-caking agents | None | Yellow prussiate of soda and similar cause foaming and staining |
The two rows at the bottom are the whole story. Iodine and anti-caking agents are added to other salt products for perfectly good reasons that have nothing to do with pools, and they are exactly what you do not want dissolved in twenty thousand gallons of water you swim in and circulate through an expensive cell.
Why the purity specification exists
It is easy to dismiss a fraction of a percent as irrelevant. The arithmetic explains why it is not.
A 20,000 gallon pool needs around 500 pounds of salt for its initial fill. If your salt is 97 percent pure rather than 99.8 percent, you have just introduced roughly 14 pounds of assorted minerals into the pool instead of about one pound. That material does not evaporate and it does not get filtered out. It stays in the water for the life of that fill, accumulating a little more every time you top up.
500 lb of 99.8% salt → about 1 lb of impurities
500 lb of 97% salt → about 15 lb of impurities
Over five years of top-ups, that difference compounds.
Iron and manganese
The most damaging impurities. Iron oxidises in chlorinated water and drops out as rust-brown staining on plaster, steps and grout. Manganese produces purple or black staining. Both are difficult to remove and often require a sequestrant plus an acid treatment.
Calcium and magnesium
They add to calcium hardness, which is already the main driver of cell scaling. Salt with high calcium content means you will be cleaning the cell more often, and every cleaning takes a little life off the plates.
Anti-caking agents
Yellow prussiate of soda keeps salt free-flowing in a bag and is entirely safe in the applications it is designed for. In a pool it can contribute to persistent foaming and cloudiness that no amount of clarifier resolves.
Staining is the failure mode to fear. Chlorine burns off, pH can be adjusted, algae can be killed. Metal staining from impure salt is permanent until treated, treatment is expensive and awkward, and in bad cases the finish needs acid washing. Saving eight dollars a bag is not worth that risk.
The three grades, and which to buy
All pool salt comes from one of three production routes, and they differ meaningfully in purity.
| Type | How it is made | Typical purity | Verdict |
|---|---|---|---|
| Mechanically or vacuum evaporated | Brine pumped from underground, evaporated under controlled conditions | 99.8–99.9% | Best. Cleanest, fastest dissolving, usually the premium bag |
| Solar salt | Seawater or brine evaporated in open ponds by sun and wind | 99.5–99.8% | Very good. The most common pool salt; excellent value |
| Mined rock salt | Halite dug from underground deposits and crushed | 95–99% | Avoid unless labelled pool grade. More insolubles and minerals |
For most owners, solar salt sold specifically as pool salt is the sensible choice: it comfortably meets manufacturer specifications, it dissolves at a reasonable rate, and it is priced accordingly. Vacuum evaporated salt is a little cleaner and a little more expensive, and is worth it if your generator specifies 99.8 percent or if you have had staining trouble before.
Rock salt is where people get into difficulty. Bags of coarse mined salt sold for de-icing or for general use often look like a bargain next to pool salt, and they are the ones most likely to carry iron and insolubles. Some rock salt is refined to pool grade and clearly labelled as such, which is fine. Unlabelled rock salt is a gamble on your pool finish.
Before you decide which grade to buy, work out the quantity. Enter your pool volume, current salt reading and target ppm into the pool salt calculator, then price the options per bag.
Grain size, and why it is on the bag
Pool salt is usually sold in a medium granular form, coarser than table salt and finer than de-icing rock salt. That is a deliberate compromise.
Finer salt has more surface area, so it dissolves faster, which matters when you are trying to distribute several hundred pounds through a pool and get an accurate reading within 24 hours. But very fine salt is dusty, blows around while you are pouring it, and clumps more readily in storage.
Coarse salt, at the other extreme, sits on the pool floor for a long time. Undissolved salt lying against plaster can etch it, and it takes a great deal of brushing to shift. Softener pellets are the extreme case: they are compressed specifically to dissolve slowly in a brine tank, which is the opposite of what a pool wants.
How pool salt actually works
The mechanism is worth understanding because it explains the single most counterintuitive fact about owning a salt pool: you barely use any salt.
Sodium chloride in water separates into sodium ions and chloride ions. At 3,000 ppm the water is about a tenth as salty as the sea, which is why it feels soft rather than briny.
The salt cell sits in the return plumbing with a stack of coated titanium plates inside. A low-voltage direct current runs between them whenever the pump is on and the generator is enabled.
The current strips electrons from chloride ions, producing chlorine, which reacts with water immediately to form hypochlorous acid. That acid is the actual sanitiser and is chemically identical to what a tablet or a jug of liquid chlorine produces.
It oxidises bacteria, algae, sweat, sunscreen and everything else swimmers introduce. This is the part that keeps the pool safe.
Having donated its oxidising power, the chlorine returns to being chloride, dissolved in the pool, indistinguishable from the salt you poured in. It will pass through the cell again in a few hours.
Salt → cell splits it → chlorine → sanitises → back to salt → through the cell again
Net salt consumed by chlorine production: essentially zero.
Salt only leaves the pool with water: splashout, backwash, overflow, leaks.
This is why a pool needs six to twelve bags on day one and then two to five a year afterwards. You are not buying fuel, you are topping up a reservoir that only leaks physically. How often to add salt to a pool covers that rhythm, and do salt water pools have chlorine goes deeper into the sanitiser side.
Pool salt compared with every other salt you might have
| Salt | Purity | Additives | Use in a pool? |
|---|---|---|---|
| Pool salt | 99.0–99.9% | None | Yes. What it is made for |
| Water softener salt (plain NaCl crystals) | 99.6–99.8% | Usually none | Often acceptable if plain and additive-free |
| Water softener pellets with additives | Varies | Rust removers, iron fighters | No. Additives cause problems |
| Potassium chloride softener salt | N/A | Different compound entirely | No. A cell cannot use it |
| Table salt, iodised | 99%+ | Iodine, anti-caking | No. Iodine discolours; absurdly expensive at volume |
| Kosher or sea salt | High | Usually none | Technically workable, wildly uneconomic |
| De-icing rock salt | 95–98% | Often anti-caking, sometimes dyed | No. Insolubles and iron risk |
| Epsom salt | N/A | Magnesium sulphate, not sodium chloride | No. Not the same substance at all |
Two entries deserve a note. Plain water softener salt is genuinely usable in many cases, and plenty of experienced owners buy it because it is cheaper per pound, but only when the bag says pure sodium chloride with no additives. The full argument, in both directions, is in pool salt versus water softener salt.
And Epsom salt appears here because people genuinely ask. It is magnesium sulphate, a completely different compound, and it will do nothing for a chlorine generator while adding magnesium to your water. It is not a salt substitute in this context despite the name.
How to read a bag before you buy it
Look for a stated purity percentage. Reputable pool salt states it plainly, usually 99.0, 99.6 or 99.8 percent. A bag with no purity claim is a bag that does not want to make one.
Check for the words “no additives” or “no anti-caking agents”. If the ingredient list mentions yellow prussiate of soda, sodium ferrocyanide, or anything described as a free-flowing agent, put it back.
Confirm it says sodium chloride, not potassium chloride. Softener aisles stock both, the bags look similar, and a cell cannot make chlorine from potassium chloride.
Check the weight, not the bag. 40 lb is the US standard but 50 lb, 20 kg and 25 kg all exist, and dosing calculations go badly wrong when you assume.
Look at the bag itself. Torn, damp or heavily caked bags have been stored badly, and a solid block of salt is a chore to break up before you can even spread it.
Cross-check your generator manual. Manufacturers specify a minimum purity and some name grades explicitly. Following it protects the warranty.
What pool salt is used for
The main job: feeding the chlorine generator
Ninety-nine percent of pool salt sold goes into pools with a salt chlorine generator, where it exists solely to be split into chlorine. Without a cell in the plumbing, adding salt to a pool achieves very little beyond changing how the water feels, since nothing is converting it into sanitiser.
The secondary effect: how the water feels
At around 3,000 ppm, dissolved salt gives water a noticeably softer, silkier feel against skin, and because that concentration is well below the salinity of tears, it does not sting eyes the way sea water does. Some owners of conventional chlorine pools add a small amount of salt purely for this effect, though it is an unusual choice and does nothing for sanitising.
The off-season use: de-icing
Because pool salt is sodium chloride, a leftover bag will melt ice on a driveway exactly like rock salt, down to about the same temperature limit. It is not a good planned winter supply since it costs more per pound than de-icing salt, but it is a perfectly reasonable use for a half bag in a storm. There are things worth knowing about concrete and plants first, covered in can you use pool salt for snow and does pool salt melt ice.
What it is not for
Pool salt is not a sanitiser on its own, not an algaecide, not a water softener for household plumbing, and not a substitute for balancing chemicals. It also should not be used as a weed killer despite the fact that it works, because the soil damage lasts for years. That is discussed in will pool salt kill weeds.
How much pool salt a pool needs
Pounds of salt = pool gallons × (target ppm − current ppm) ÷ 120,000
Example: 20,000 gal, currently 500 ppm, target 3,200 ppm
20,000 × 2,700 ÷ 120,000 = 450 lb, which is about eleven 40 lb bags
| Pool volume | Initial fill to 3,000 ppm | 40 lb bags | Typical annual top-up |
|---|---|---|---|
| 5,000 gallons | 125 lb | 3 bags | 1–2 bags |
| 10,000 gallons | 250 lb | 6 bags | 1–3 bags |
| 15,000 gallons | 375 lb | 9 bags | 2–4 bags |
| 20,000 gallons | 500 lb | 12 bags | 2–5 bags |
| 30,000 gallons | 750 lb | 18 bags | 3–6 bags |
Always subtract what is already in the water. Pools that have been sanitised with liquid chlorine for years often sit at 500 to 1,000 ppm before you add anything, because liquid chlorine leaves salt behind as it is consumed. Test first using the methods in how to test salt level in a pool, and aim slightly under target so you can top up rather than having to dilute. The method for getting it into the water is in how to add salt to a pool the right way.
Storing pool salt, and whether it goes off
Sodium chloride is a mineral. It does not expire, degrade, oxidise or lose potency. A sealed bag stored properly will be exactly as good in five years as it was on the day you bought it.
What does go wrong is physical. Salt is hygroscopic, meaning it pulls moisture out of the air, and a partially used bag left open in a humid shed will fuse into a solid block that takes real effort to break apart. Worse, a bag sitting directly on a damp concrete floor can wick moisture up from below and pick up rust from any nearby metal, which reintroduces exactly the iron contamination the purity spec exists to avoid.
Seal partial bags. Fold the top over and tape it, or decant into a lidded bucket. Ten seconds of effort saves a caked block next spring.
Keep bags off the floor. A pallet, a shelf or a couple of lengths of timber is enough. Concrete floors wick moisture.
Store away from rusting metal. Salt and iron are a bad combination, and you do not want rust flakes ending up in the bag.
Caked salt is still usable. Break it up and use it, as long as it is clean. Discoloured, gritty or rust-marked salt should go on the driveway instead.
Mistakes people make with pool salt
Buying on price without checking purity. The bag that saves you five dollars can cost hundreds in stain removal.
Using iodised table salt or de-icing rock salt. Iodine discolours, and rock salt brings insolubles and iron.
Grabbing potassium chloride softener salt. Same aisle, similar bag, completely useless to a chlorine generator.
Assuming the pool starts at zero salt. Most do not. Test first or you will overshoot and have to drain water to correct it.
Pouring salt into the skimmer. Sends a concentrated slug through the pump, heater and cell. Broadcast it into the pool instead.
Not brushing after adding it. Undissolved salt sitting on plaster can etch the finish, and it delays an accurate reading.
Testing the same day. Give it a full 24 hours of circulation before you believe any number.
Pool salt: common questions
What is pool salt?
Pool salt is sodium chloride refined to at least 99 percent purity with no additives, sold in granular form for dissolving in swimming pool water. It is the same compound as table salt and rock salt, but without the iodine, anti-caking agents and mineral impurities those products contain. A salt chlorine generator uses it as raw material, splitting it electrically to produce chlorine continuously inside the pool plumbing.
What is in pool salt?
Almost entirely sodium chloride, typically 99.0 to 99.9 percent. The small remainder is trace moisture and tiny quantities of calcium, magnesium, sulphate and insoluble matter left over from processing. Good pool salt contains no iodine, no anti-caking agents such as yellow prussiate of soda, and no free-flowing additives, because all of those cause problems in a pool ranging from staining to persistent foaming.
How does pool salt work?
Dissolved salt separates into sodium and chloride ions in water. When pool water flows through the salt cell, a low-voltage current splits the chloride and produces chlorine, which immediately becomes hypochlorous acid, the same sanitiser found in any chlorine pool. Once that chlorine has oxidised whatever it found, it reverts to chloride, ready to pass through the cell again. Salt is recycled rather than consumed.
Is pool salt the same as table salt?
Chemically the base compound is identical, but table salt is not suitable for a pool. Most table salt is iodised, and iodine can discolour water and contribute to staining. It also usually contains anti-caking agents that can cause cloudiness and foaming, and its very fine grain makes it awkward to broadcast. On top of that, buying enough table salt to fill a pool would cost many times more than pool salt.
What is the purity of pool salt?
Look for 99.0 percent sodium chloride as an absolute minimum and 99.8 percent or higher if the option is there. Most manufacturers of salt chlorine generators specify a minimum purity in their manual, and using anything below it can void a warranty. The purest product is mechanically or vacuum evaporated salt at around 99.9 percent, followed by solar salt, with mined rock salt the least pure.
What is pool salt used for?
Its primary purpose is supplying a salt chlorine generator with the raw material it needs to make chlorine on site. A secondary effect is the feel of the water, since salt at around 3,000 ppm gives a softer sensation on skin and is easier on the eyes than a conventional chlorine pool. Outside the pool, people occasionally use leftover bags for de-icing a driveway, where it works exactly like rock salt.
Can you use water softener salt in a pool?
Sometimes, but only if it is plain sodium chloride at 99 percent or higher with no additives, which some softener salt is and some is not. Many softener pellets contain rust removers, iron-fighting additives or anti-caking agents, and some are potassium chloride rather than sodium chloride, which a salt cell cannot use. Pellets also dissolve very slowly. Check the bag carefully rather than assuming.
Does pool salt expire or go bad?
Sodium chloride does not expire. What can go wrong is physical rather than chemical: salt absorbs moisture from the air and cakes into a solid block, and a bag left on a damp concrete floor can pick up rust and grit that you do not want in a pool. Stored sealed, off the floor and somewhere dry, an unopened bag will still be perfectly usable years later.
What size bag does pool salt come in?
In the United States 40-pound bags are the standard, with 50-pound bags available from some suppliers. Elsewhere 20 kg and 25 kg bags are typical. Since dosing calculations are usually expressed in pounds or kilograms rather than bags, check the weight on the bag rather than assuming, because a calculation done in 40-pound bags will be significantly off if you actually bought 25 kg ones.
How much pool salt does a pool need?
For an initial fill from near zero, roughly 250 pounds per 10,000 gallons brings a pool to about 3,000 ppm, so six 40-pound bags for a 10,000 gallon pool and twelve for a 20,000 gallon one. After that you only replace what physically leaves the pool, which is usually two to five bags a year. Always test your existing salt level first and calculate from there rather than assuming you are starting at zero.
Is pool salt safe to touch and handle?
Yes, it is ordinary salt and far less hazardous than most pool chemicals. It will dry and irritate skin on prolonged contact, sting any cuts, and irritate eyes, so gloves and sensible handling are reasonable, but there is nothing here comparable to muriatic acid or chlorine products. The main practical hazard is the weight of the bags, since a 40-pound bag lifted awkwardly is a good way to hurt your back.
Why is my pool salt not dissolving?
Usually one of three reasons. The grain is too coarse, which happens if you bought de-icing crystals or softener pellets rather than pool-grade salt. The pump is not running, so there is no circulation to carry dissolved salt away from the pile. Or the water is cold, which slows dissolution considerably in spring and autumn. Brushing the salt across the floor rather than leaving it in a heap fixes most cases.
The quick version
Pool salt is sodium chloride at 99 percent purity or better with nothing added. What makes it a distinct product is what has been removed: iodine, anti-caking agents, iron, manganese and insolubles, all of which cause staining, cloudiness, foaming or scaling in a pool. Buy solar or evaporated salt labelled for pools, check the purity figure on the bag, and match whatever minimum your generator manual specifies.
It works by dissolving into the water so a cell can split it into chlorine, and because that chlorine reverts to salt after sanitising, you barely consume any. Six to twelve bags on day one, two to five a year afterwards, replacing only what physically leaves with the water.
Next: how much salt to add, how to add it properly, what level to aim for, where to buy it, whether softener salt works and how many bags a startup takes. Run your numbers in the free pool salt calculator, browse more tools at waldev.com, and see what else we build at waldev.co.
Accuracy note: purity grades, bag sizes and dosing figures here are typical values and vary by manufacturer and region. Always follow the salt specification given in your own salt chlorine generator manual, since using a product below the stated purity can affect warranty cover. Test your water before dosing rather than relying on general figures.
The USGS minerals programme documents how salt is mined, solar evaporated and vacuum processed, and the purity each route achieves. USGS salt statistics →
Calcium and magnesium carried in by impure salt add to hardness, the main driver of cell scaling. USGS water hardness →
CDC Healthy Swimming explains the chlorine residual a pool needs, whichever way it is produced. CDC Healthy Swimming →
The Pool & Hot Tub Alliance publishes the consensus water chemistry standards used across the industry. Pool & Hot Tub Alliance →
