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A two by four has not measured two inches by four inches since your grandfather was buying them

Most people know this. Far fewer know the exact figures, that they are set by a federal standard rather than by habit, that green lumber is legally larger than dry lumber, or that the rule people carry in their heads breaks at eight inches wide.

The document is PS 20-25, the American Softwood Lumber Standard, published in January 2025 by the National Institute of Standards and Technology for the United States Department of Commerce. It supersedes PS 20-20 Revision 1 of October 2021. Almost every lumber page online, including several trade sites, still cites an older edition.

Both tables are the minimum dressed sizes in PS 20-25, in inches. The standard adds one line that saves a lot of confusion: the thicknesses apply to all widths and all widths apply to all thicknesses.

The rule breaks at eight inches, and that is where the arithmetic goes wrong

Everybody learns that a nominal dimension loses half an inch. That is true up to nominal 6 and false from nominal 8 upward, where the loss becomes three quarters of an inch. A 2x12 is 11-1/4 inches wide, not 11-1/2. On a single joist that is a quarter inch. Across a floor laid out on the wrong figure it is a cumulative error that shows up as a gap at the last board, and it is the single most common arithmetic mistake in do-it-yourself material estimating.

Green lumber is legally bigger, and that is deliberate

PS 20-25 defines the two states precisely. Dry Lumber is lumber of less than nominal 5-inch thickness which has been seasoned or dried to a maximum moisture content of 19 percent. Green Lumber is lumber of less than nominal 5-inch thickness which has a moisture content in excess of 19 percent. Nineteen percent is the line, and it is written into the definitions rather than left to judgment.

Green stock is cut oversize because it is going to shrink. A green 2x4 leaves the mill at 1-9/16 by 3-9/16 so that when it dries it lands inside the dry tolerance. Which means two boards labeled 2x4, bought on the same day from the same yard, can legitimately differ by a sixteenth of an inch in each direction depending on how they were dried. Note also that both definitions stop at nominal 5-inch thickness. Timbers of nominal 5 inches and thicker sit under a different part of the standard with a different table, so a 6x6 is not governed by the rule above.

The guide to linear, square and board feet covers what happens when these dimensions feed a conversion, and the board foot calculator does the arithmetic once you have decided which set of dimensions to use.

In roughly 46 states it is illegal to label a board 2x4 without showing the real size nearby

This is the strongest thing on this page and almost nobody knows it. The nominal size on a piece of lumber is not a courtesy or a trade habit. It is permitted by a weights and measures regulation, on conditions, and the conditions are specific.

The document is NIST Handbook 130, Uniform Laws and Regulations in the Areas of Legal Metrology and Fuel Quality, Section IV.B, the Uniform Regulation for the Method of Sale of Commodities. The current edition is Handbook 130 for 2026, adopted by the 109th National Conference on Weights and Measures. Section 2.10 covers softwood lumber and section 2.12 covers hardwood lumber sold at retail.

Softwood: sold by piece and surfaced size, not by board measure

Under section 2.10, softwood quantity has to be stated as the number of pieces, the minimum surfaced width and thickness, and either the length of individual pieces or the lineal footage. Nominal dimensions may be used, but only if two things are both true:

1. the term nominal or nom is also used; and
2. the actual or minimum dressed sizes are prominently displayed to the customer, either by means of a table or a label.

The regulation gives its own worked example, in the text: a dry 2 x 4 is surfaced to the actual dimensions of 1-1/2 in by 3-1/2 in. That sentence is in the regulation, not in a guide to the regulation.

The American Wood Council, which represents the manufacturers, states that approximately 46 states have adopted the Uniform Regulation for the Method of Sale of Commodities, and describes the rule in plainer language: a 2x4 can be labeled a 2x4, but only if the label or other displays notes that 2x4 is the nominal size and prominently features the actual or minimum measurements. On paperwork, its guidance is that sellers should show invoices by actual sizes for standard sized lumber and dressed sizes for nonstandard sized lumber. The forty-six figure is the characterization of the council itself rather than a NIST count, which is why it carries the word approximately here.

Hardwood: sold by the board foot, and the board foot is defined

Section 2.12 handles hardwood differently, and here the board foot is the legal unit. It is defined as the volume of a board 1 ft long, 1 ft wide, and 1 in thick or its equivalent, which the regulation states as 144 cubic inches of wood. The same nominal-disclosure condition applies: nominal dimensions are allowed only where the table of minimum surfaced sizes for kiln dried hardwood lumber, or the actual dimensions, are prominently displayed and the term nominal or nom is used.

So the answer to whether a board foot uses nominal or actual dimensions is that it depends on the species group, and the law splits them. Softwood is not sold by board measure at retail at all. Hardwood is, and hardwood board footage is conventionally computed on the rough size before surfacing.

Which is where the money actually goes

The American Hardwood Export Council publishes the surfacing losses. For hardwood up to and including 1-1/2 inches, subtract 3/16 inch from the nominal thickness. For 1-3/4 inches and above, subtract 1/4 inch. So four-quarter hardwood, nominally one inch thick, surfaces to 13/16 of an inch, and the buyer paid board footage on the full inch.

Thickness losses are the published AHEC figures. The percentages are arithmetic performed here on those figures, not published values, and they assume board footage is tallied on the rough size, which is the convention rather than a legal requirement.

Two federal definitions sitting one page apart, not reconciled

There is a second federal definition of a board foot, in an agricultural marketing rule. Title 7 of the Code of Federal Regulations, part 1217, which runs the softwood lumber research and promotion program, defines a board foot at section 1217.4 as a board 12 inches long, 12 inches wide, and 1 inch thick or its cubic equivalent. Ten sections later, at 1217.16, the same rule defines nominal size as the size by which softwood lumber is known and sold in the marketplace that differs from actual size.

One rule, two definitions, no reconciliation between them. The government defines the unit as a real volume of wood and, in the same document, records that the sizes lumber is known and sold by are not the actual sizes. Neither definition is wrong. They simply describe two different things that share a vocabulary, which is the theme of this entire page.

Three plumbing materials on the same aisle, three unrelated ways of not telling you the diameter

Lumber and panels are at least consistent about being smaller than the label. Pipe is stranger: a half inch pipe is larger than half an inch on the outside and smaller than half an inch on the inside, and neither figure is 0.500.

The system is called Nominal Pipe Size, and the governing document for steel is ASME B36.10, Welded and Seamless Wrought Steel Pipe. The values below are read from three manufacturers who publish full dimension tables, because the ASME table itself sits behind a paywall. Three independent manufacturers in two different materials agree exactly, which is the point of the system: the outside diameter is fixed by the nominal size, and the wall thickness, called the schedule, eats into the bore.

Outside and inside diameters in inches, from published Schedule 40 submittal sheets. The same outside diameters appear in a steel fire sprinkler pipe data sheet and a PVC drain pipe submittal package, confirming that the outside diameter belongs to the nominal size rather than to the material. Percentages are arithmetic performed here.

The rule stops abruptly at 14 inches

Below nominal 14 inches, the label never equals the outside diameter. At nominal 14 and above, it always does, exactly. A 14 inch pipe has a 14.000 inch outside diameter, a 16 inch pipe measures 16.000, and so on upward.

There is no gradual convergence toward the label as sizes increase. It is a hard discontinuity, and it is a fossil: the small sizes were built around nineteenth century iron pipe wall thicknesses and inherited those outside diameters, while the large sizes were specified later and simply took the sensible option.

Copper is a third system again

Copper water tube does not follow Nominal Pipe Size. The Copper Development Association states the rule for Types K, L and M tube in one sentence: in each case the actual outside diameter is 1/8 inch larger than the nominal or standard size. So half inch copper tube has an outside diameter of 0.625 inch, three quarter inch is 0.875, and the offset stays the same at every size instead of shrinking proportionally the way it does for steel and plastic.

Add ten percent for waste. Nobody can tell you where that number came from

Every material estimate ends with the same instruction, and it is worth pulling on. Ten percent for waste, fifteen for a diagonal tile layout, twenty for a pattern repeat. It appears in retailer guides, contractor blogs and every online calculator. It does not appear in any standards document, manufacturer instruction or extension publication that could be found.

Here is what was checked, and what came back.

Why the ten percent rule is suspect on its own terms

Set aside the missing source for a moment. The rule is size-independent, and waste is not. What generates offcuts is cut density: the number of edges, the shape of the room, whether the layout runs diagonally, how long the pattern repeat is, how many hips and valleys a roof has. A long rectangular room and an identically sized room with three doorways, a bay and a chimney breast do not waste the same fraction, and a percentage of total area cannot know the difference.

That does not make ten percent a bad working figure. It makes it a rule of thumb, and the honest thing is to say so rather than dress it up as a standard. If the layout is simple, less will do. If it is complicated, ten percent will not be enough, and the number that matters is whether the supplier still has the same dye lot in three weeks. The flooring measurement guide uses the ten percent convention, which is the sensible working choice; this section is about where it does and does not come from.

The one measured figure that exists, and its age

The closest thing to real data is a measurement rather than a recommendation. A characterization of building-related construction and demolition debris prepared for the Environmental Protection Agency, EPA530-R-98-010, published in June 1998, gives a weighted average for new residential construction of 4.38 pounds of debris per square foot. For residential renovation it reports individual project assessments ranging from 2.41 to 11.3 pounds per square foot, and notes that remodeling typically generates more waste per square foot than new construction, recommending estimation by improvement type rather than by a single per-square-foot rate.

Two cautions. That report is from 1998, so it predates a great deal of panelization and prefabrication, and it should not be presented as a current figure. And it measures debris leaving a site, which includes packaging, demolition material and damage, not only offcuts from new material. It is the best documented thing available and it is still not an answer to how much extra to order.

For current scale rather than per-project rates, the EPA estimated that 600 million tons of construction and demolition debris were generated in the United States in 2018, on a page last updated in October 2025.

Four collections, and which one you need

Each has its own hub covering the detail this page leaves alone. Counts are current as of 19 August 2026.

The tools

• Board Foot CalculatorThickness, width, length, quantity and price per board foot.
• Square Feet CalculatorArea in square feet, square yards and square meters.
• Topsoil Calculator and the Cubic Yard CalculatorCubic feet, cubic yards, tons and bag counts from area and depth.
• Pool Salt CalculatorSalt required from pool volume, current reading and target level.
• Mulch CalculatorNote the extra material dropdown, which offers 5, 10, 15 or 20 percent. Section 08 is about where those numbers come from.
• All home calculatorsEighteen tools, including paint, asphalt, stairs and repair costs.

The other eleven sections of the site are on the blog hub.

The documents behind everything above

Every figure on this page is tied to the body that publishes it, with the edition and date, because standards in this area are revised on cycles and a number quoted without its edition cannot be checked. All links verified 19 August 2026.

• Softwood lumber dimensions. PS 20-25, American Softwood Lumber Standard, NIST for the US Department of Commerce, January 2025, which supersedes PS 20-20 Revision 1 of October 2021. The full text is hosted by the American Lumber Standard Committee, whose PS 20 page confirms it is current. Source of the dressed size tables, the dry and green definitions, the 19 percent line and the statement on voluntary standards.
• Nominal sizing, cross-checked. Wood Handbook, chapter 5 on commercial lumber, USDA Forest Service Forest Products Laboratory, FPL-GTR-113, which records that lumber length is given in actual dimensions while width and thickness are traditionally recorded in nominal ones.
• How the 2x4 shrank. Smith and Wood, History of Yard Lumber Size Standards, Forest Products Laboratory, USDA Forest Service, September 1964, source of the 1924 standard at 1-5/8 inches, the 1956 and 1957 defeat of the reduction proposal, the December 1961 Scottsdale agreement and the 1-1/2 inch figure carried into the 1964 revision. The only fetchable copy is a scan on a commercial host rather than a government domain, which is noted here rather than hidden. Edition lineage from the Federal Register: PS 20-94 replacing PS 20-70, 31 March 1994 and the five-year review notice of 6 March 2024.
• The committee. American Lumber Standard Committee history, source of the 1922 start, the 1924 first publication and the 1953 court reference. The case behind that reference is not identified on the page and is not cited here.
• How lumber must be labeled and sold. NIST Handbook 130, current edition, and Section IV.B of the 2026 edition, the Uniform Regulation for the Method of Sale of Commodities. Source of section 2.10 on softwood, 2.12 on hardwood and the 144 cubic inch board foot, 2.7 on roofing and the definition of a square, 2.3 on coatings, 2.14 on insulation, 2.18 on mulch, 2.5 on peat and 2.9 on sod, plugs and sprigs. State adoption illustrated by Nebraska, which carries the same section numbers.
• The labeling rule in plainer words, and the state count. American Wood Council, weights and measurement, source of the approximately 46 states figure, the invoicing guidance and the advice against labeling in actual sizes only. That state count is the trade body describing itself and is not a NIST tally.
• The federal board foot definitions. 7 CFR part 1217, subpart A, defining a board foot at section 1217.4 and nominal size at section 1217.16.
• Hardwood surfacing losses. American Hardwood Export Council on measurement, giving the board foot formula applied to rough sawn lumber and the 3/16 and 1/4 inch surfacing deductions. The plain-language definition and the formula also appear in the NC State Extension forestry glossary.
• General units. NIST Handbook 44, 2024, Appendix C, general tables of units of measurement, for the gallon at 231 cubic inches, the cubic foot at 7.481 gallons and the cubic yard at 27 cubic feet exactly. The board foot does not appear in it.
• Panels. PS 2-18, Performance Standard for Wood Structural Panels, source of the Performance Category thickness ranges and the plus or minus 1/32 inch tolerance, and PS 1-19, Structural Plywood, effective 1 December 2019, source of the Performance Category definition. Dual labeling and the 2008 date from the Engineered Wood Association on OSB and on plywood, and the tolerance collision from its technical topic on panel thickness tolerances and labeling.
• Pipe. ASME B36.10M, welded and seamless wrought steel pipe. The dimension table itself is behind a paywall, so the figures on this page come from three manufacturers who publish theirs: JM Eagle Schedule 40, Wheatland Tube Schedule 40 and Charlotte Pipe PVC Schedule 40 DWV, which is also the source for the discontinuity at nominal 14. Copper from the Copper Development Association.
• Yields and coverage. Quikrete concrete mix data sheet, product 1101, revised October 2022. Paint from the published data page for Sherwin-Williams ProMar 200 Zero VOC Interior Latex Eg-Shel, revision March 2025. Shingles from GAF Timberline HDZ, GAF Camelot II and GAF Grand Sequoia.
• Waste, and the search that came back empty. National Wood Flooring Association standards for factory finished solid wood flooring, whose only 5 percent figures are manufacturing tolerances rather than ordering guidance. The measured debris figures are from Characterization of Building-Related Construction and Demolition Debris in the United States, EPA530-R-98-010, June 1998, which is 28 years old, and the national total from EPA construction and demolition debris data, updated October 2025.
• Spending. Joint Center for Housing Studies of Harvard University, Leading Indicator of Remodeling Activity, released 5 May 2026, and US Census Bureau monthly construction spending for June 2026, released 3 August 2026.
• Pools. CDC Model Aquatic Health Code, fourth edition, February 2023, for the definition of turnover, and CDC guidance on home pool and hot tub water treatment and testing, reviewed 10 May 2024, for the chlorine minimums and the pH range.
• Noted and not published. Several things were checked and deliberately left out. No published waste or overage percentage could be found from any standards body, manufacturer or extension service, so none is quoted. The historical reason plywood settled on 15/32 is not in either panel standard, so the usual explanations are not repeated. Drywall is not claimed to be exactly its nominal thickness, because the tolerance lives in a paywalled material standard that could not be read. No claim is made about timbers of nominal 5 inches and thicker, which sit under a different table. The exact date the 1-1/2 inch lumber thickness took commercial effect is not established in the 1964 report, so it is described as agreed in 1961 and carried into the 1964 revision rather than as a 1964 change. And the pipe dimensions are attributed to the manufacturers who publish them rather than to ASME, whose table was not readable.