Board foot calculator

A board foot is a plank one inch thick, one foot wide and one foot long, and counting them is one multiplication. The part that catches people is which inch. A "2 by 4" has not been two inches by four since it left the saw: the American Softwood Lumber Standard says outright that a dry one is surfaced to 1 1/2 by 3 1/2 inches, and that the same is true all the way up the table. Board feet are counted on the label size anyway, because that is how the standard defines the tally and how a yard writes its invoice. Cubic metres are counted on the dressed size, because that is the wood. Both numbers are correct and they differ by a third on a 2 by 4, so this calculator shows you both and tells you which size it used for each. Say whether you are typing the label or a tape measurement and it does the rest.

How it is calculated

board feet = nominal thickness (in) × nominal width (in) × length (ft) ÷ 12

PS 20 puts it as thickness in inches times width in feet times length in feet; dividing the width in inches by twelve is the same sum with fewer conversions. A nominal 2 by 4 eight feet long is 2 × 4 × 8 ÷ 12 = 5.33 board feet. The cubic figure is a different sum on different numbers: the dressed 1 1/2 by 3 1/2 inches over 96 inches of length is 504 cubic inches, or 0.00826 m³, where the nominal box would have been 768 cubic inches. That gap is not an error in either figure — it is the planer shavings, and PS 20 warns about converting across it in as many words.

Source: NIST Voluntary Product Standard PS 20-20 Revision 1 (October 2021), American Softwood Lumber Standard — 2.2 Board measure — "The number of board feet in a piece of lumber is obtained by multiplying the nominal thickness in inches or fraction of an inch by the nominal width in feet by the length in feet"

Questions people ask

Why is a 2 by 4 not two inches by four inches?

Because the nominal size is a name for a size category, not a measurement. PS 20 defines it that way: "The nominal size is greater than the dressed size i.e., a dry 2 by 4 is surfaced to 38.1 mm by 88.9 mm (1 1/2 by 3 1/2 inches)." The board is sawn oversize green, then dried and planed smooth, and both steps take material off. The label kept the sawn size. Table 3 of the standard lists the whole ladder: nominal 1 dresses to 3/4, nominal 6 in width to 5 1/2, nominal 12 to 11 1/4.

So which size does a board foot use?

The nominal one. PS 20 section 2.2 says the number of board feet "is obtained by multiplying the nominal thickness in inches or fraction of an inch by the nominal width in feet by the length in feet". That is not a rounding convenience — it is how lumber is bought and sold, so the tally on your invoice will use it whatever your tape says. Choose the measured option here only when you want to know how much wood is really in the pile, and expect a smaller number than the yard's.

Can I convert board feet to cubic metres?

Only if you say which size you meant, which is why this tool does the two sums separately. The standard is blunt about it: "CAUTION: Use great care when converting board feet, based on NOMINAL cross-sectional dimensions, to cubic meters of lumber, based on DRESSED cross-sectional dimensions." It quotes IEEE/ASTM SI 10 in support: "No conversion is given for board foot because board foot is not a well-defined unit of volume." A single multiplier between the two exists only for one particular cross-section.

What about anything five inches or thicker?

That is a timber, and PS 20 stops using a lookup table for it. Section 3.4.3 defines timbers as lumber of nominal 5-inch or greater least dimension, and Table 3 gives them a flat allowance instead: half an inch off at nominal 5 and 6, three quarters off from 7 to 15, a full inch at 16 and above, applied to thickness and width separately. So a 6 by 6 dresses to 5 1/2 square and a 16 by 16 to 15 square — and note that a 16-inch-wide board dresses to 15 1/4, a quarter inch wider than the timber of the same nominal width. This tool applies the timber rule when both dimensions reach five inches, exactly as the standard scopes it.

Does the dry or green column apply to what I bought?

Dry, almost certainly, and that is what this tool uses. PS 20 calls lumber dry when it has been seasoned to 19 percent moisture content or less, which covers essentially everything kiln-dried you will find stacked indoors at a yard. Green lumber is dressed slightly fatter to allow for the shrinkage still to come — a green 2 by 4 leaves the mill at 1 9/16 by 3 9/16 — and it is mostly timbers of nominal 5 inches and up that are sold that way, because they are often never dried at all. If your stock is genuinely green, add a sixteenth to each dressed figure.

Is this standard the one that applies where I live?

It applies to softwood lumber sold in the United States. PS 20 is a Voluntary Product Standard published by NIST under the Department of Commerce, and the sizes in it are the ones the American trade runs on. Other countries size lumber their own way and often sell it by the cubic metre rather than the board foot, in which case the second figure here is the one you want and the nominal question never arises. The board foot itself is used beyond the United States, but the dressed sizes behind it are not.

Where do the numbers on this page come from?

One document, and you can read it yourself: Voluntary Product Standard PS 20-20 Revision 1, October 2021, the American Softwood Lumber Standard, published by NIST for the U.S. Department of Commerce, at https://www.nist.gov/system/files/documents/2021/10/26/PS%2020-20%20Revsion%201%20October%202021.pdf — section 2.2 for the board-foot tally, 2.16 for what a nominal size is, 3.4.3 for timbers and Table 3 for every dressed size this tool uses. Nothing here is averaged, estimated or remembered.

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