Lumber · Guide

What is a board foot?

A board foot is a unit of lumber volume: 144 cubic inches, the wood in a piece 1 ft × 1 ft × 1 in. The count is simple arithmetic. What it hides is how much of that board foot is actual wood, and how a hardwood yard really tallies it. Imperial or metric.

Detail A — Board feet, both ways

Board feet — by the definition

— board feet (nominal)

Actual wood you receive — surfaced (PS 20)

Surfaced size (in)—

Actual volume (ft³)—

Share that is wood—

01 How the math works

A board foot measures volume, not length or area. One board foot is the wood in a piece a foot long, a foot wide, and an inch thick — 144 cubic inches, or one-twelfth of a cubic foot. It is the unit lumber is priced and tallied in once a board is thicker than a sheet of veneer.

To count board feet from a board's size, multiply the three dimensions and divide out the unit mismatch:

Board feet = T(in) × W(in) × L(ft) ÷ 12
1 board foot = 144 in³ = 1/12 ft³
Surface measure (hardwood) = W(in) × L(ft) ÷ 12, rounded to nearest ft²

The ÷ 12 is there because thickness and width are in inches while length is in feet; it turns the mixed units back into the 144-cubic-inch block. Enter the dimensions the board is called — the nominal size — not the size a tape measure reads on a finished board. That distinction is the whole reason this page has two more sections: the number the yard charges you for is figured on nominal size, and a fair amount of that nominal size has been planed away by the time the board reaches you.

02 Worked example

Take a framing order of 200 linear feet of 2×8 (61 m of 38×184 mm). Board feet is length times the nominal cross-section over twelve: 200 × (2 × 8) ÷ 12 = 266.67 bf. That is the figure the invoice is built on, and it is exactly right as arithmetic.

Now weigh it against the wood. A surfaced 2×8 is 1.5 × 7.25 in (PS 20), so the real timber in that order is 15.10 ft³ (0.428 m³) — against the 22.22 ft³ (0.629 m³) the 266.67 nominal board feet would imply. About 68% of the board footage you paid for is wood; the rest is the difference between what a 2×8 is called and what it is.

03 When the board-foot count misleads

The formula is exact. What it does not tell you is how much wood a board foot is, or how the yard arrived at the number on the ticket. Two gaps do the damage.

Softwood board feet are figured on nominal size, not the surfaced size. A 2×8 board foot is billed on a 16 in² cross-section, but a dressed 2×8 is 1.5 × 7.25 = 10.875 in² — 68% of nominal. The nominal-versus-actual gap is the single biggest source of estimating errors in DIY framing, and it is not an opinion so much as PS 20: a 5/4 deck board dressed to 1.0 in is 20% thinner than the thickness its board-foot count assumes. Order and lay out by the actual dimensions; use board feet only to compare prices.
Rough hardwood is not tallied with this formula at all. Hardwood yards use surface measure — width in inches × length in feet ÷ 12, rounded to the nearest whole square foot — times the nominal thickness in quarters (4/4 = 1 in tallies one board foot per surface foot, 8/4 = 2 in tallies two). A 4/4 board 7 in × 8 ft has a surface measure of 4.67 → 5 ft², billed as 5 board feet against an exact 4.67 bf, about 7% over. A board whose surface measure rounds down bills under. The clean formula and the yard's tally rarely land on the same number for a single board.
Hardwood is priced on the rough thickness, before it is surfaced. The standard surfaced-two-sides thickness of 4/4 lumber is 13/16 in (0.8125 in / 20.6 mm). Roughly 3/16 in is planed off, yet the board still bills as a full 4/4 board foot. If your parts finish thinner than 13/16, buy 4/4 rough and mill it yourself; if you need it flat and ready, pay for surfaced stock and the footage it was tallied at.
Board feet say nothing about how the wood is distributed. A hundred board feet of 4/4 and a hundred board feet of 8/4 are the same volume, but a different number of boards, a different price per board foot, and different yield after you mill to a finished thickness. Match the thickness to the part before you compare footage.

04 What to do with the result

For softwood framing and decking, board feet are a pricing unit, not a build quantity. Nobody builds with 266 board feet; they build with a stack of 2×8s of a given length. Use the board-foot count to compare a per-board-foot quote against a per-piece shelf price, and use the actual PS 20 dimensions to lay out studs, joists, and decking. The two live on different pages of the same order.

For hardwood, expect the invoice to be figured by surface measure on the rough thickness, and check the yard's tally with your own surface-measure count rather than the clean formula — that is the number that will match theirs. Decide up front whether you are buying rough 4/4 to mill down or surfaced stock ready to use, because you pay for the rough footage either way.

05 Common questions

What is a board foot?: A board foot is a unit of lumber volume equal to 144 cubic inches — the wood in a board 1 ft long, 1 ft wide, and 1 in thick. It is figured as nominal thickness (in) × nominal width (in) × length (ft) ÷ 12. It is the standard way rough and dimensional lumber is priced and tallied.
How do you calculate board feet?: Multiply nominal thickness in inches by nominal width in inches by length in feet, then divide by 12. One 2×8 that is 8 ft long is (2 × 8 × 8) ÷ 12 = 10.67 bf. Multiply by the number of pieces for a run. The calculator above does it in imperial or metric.
Is a board foot the same as a cubic foot?: No. A board foot is 144 in³ and a cubic foot is 1,728 in³, so a board foot is exactly one-twelfth of a cubic foot — 12 board feet = 1 ft³. The board foot exists because lumber is thin relative to its length and width; the cubic foot would give awkward fractions for a single board.
What is the difference between a board foot and a linear foot?: A linear foot is a length only; a board foot is a volume that also counts thickness and width. One linear foot of 2×8 is 1.33 board feet, while a linear foot of 1×4 is 0.33 board feet — same length, very different volume. Softwood at the home center is often priced per piece or per linear foot; hardwood is priced per board foot.
What does 4/4 mean in lumber?: It is rough hardwood thickness written in quarters of an inch: 4/4 = 1 in, 5/4 = 1.25 in, 6/4 = 1.5 in, 8/4 = 2 in. The board foot is figured on that rough thickness. A 4/4 board dresses to a standard 13/16 in surfaced, but still bills as a full 4/4 board foot.