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What Is a Board Foot?

What is a board foot? It is the lumber yard's way of charging for volume: one board foot is a slab 1 foot square and 1 inch thick, or 144 cubic inches of wood in any shape that multiplies out the same. A long thin board and a short thick one can hold the same board feet — and the same price — even though they look nothing alike.

By the UrbiTools editorial team · Rates checked against GOV.UK · Updated

The formula

Board feet = (thickness in inches × width in inches × length in feet) ÷ 12. The division by 12 converts the inch-foot mix into cubic feet divided by twelve — since one board foot is one-twelfth of a cubic foot. Three examples show the range: a 1×6 board 10 feet long is (1 × 6 × 10) ÷ 12 = 5 board feet; a 2×8 board 12 feet long is (2 × 8 × 12) ÷ 12 = 16 board feet; and a chunky 4×4 post 8 feet long is (4 × 4 × 8) ÷ 12 = 10.67 board feet.

Why lumber is sold this way

Pricing by the foot of length would punish thick boards and reward thin ones; pricing by weight would punish dense hardwoods. Board feet price the actual material, so a walnut board and a pine board of the same dimensions cost in proportion to their per-board-foot rates, and a project estimate is just total board feet times the rate, plus waste. Most yards add 10–15% for offcuts and defects when quoting a project — a habit worth copying, since no board is usable end to end.

The nominal-size trap

Shelf labels are nominal: a 2×4 is sawn 2 × 4 inches rough, then planed smooth to about 1.5 × 3.5 inches, but it keeps the 2×4 name and the board-foot math uses the nominal figures. That means the 5.33 board feet of an 8-foot 2×4 describes the rough-sawn volume, not the planed board you take home. For structural framing this hardly matters — everyone prices the same way — but if you are comparing a rough-sawn hardwood supplier against a planed softwood yard, check which dimensions each quote is based on before comparing per-board-foot prices.

For firewood, a different volume unit rules: see what a cord of wood is, and for the length and area conversions behind any project, start with the feet to metres converter.

Sources & official guidance

Frequently asked questions

What is a board foot?
A board foot is a unit of lumber volume: a piece 1 foot long, 1 foot wide and 1 inch thick, equal to 144 cubic inches (about 2,360 cubic centimetres). Lumber is commonly priced per board foot, so the unit is how you compare the cost of different boards.
How do you calculate board feet?
Multiply thickness in inches × width in inches × length in feet, then divide by 12. A board 2 inches thick, 6 inches wide and 8 feet long is (2 × 6 × 8) ÷ 12 = 8 board feet.
Is a board foot the same as a square foot?
No. A square foot measures area; a board foot measures volume and includes thickness. A 1-inch board covering 1 square foot is 1 board foot, but a 2-inch board covering the same area is 2 board feet.
Do board feet use nominal or actual lumber sizes?
Nominal sizes — the names on the shelf, like a 2×4. A 2×4 is actually about 1.5 × 3.5 inches after planing, but board-foot calculations traditionally use the nominal 2 × 4 dimensions, which is why the same board can seem to contain more volume than it physically does.
How many board feet are in a 2×4 stud 8 feet long?
(2 × 4 × 8) ÷ 12 = 5.33 board feet by nominal size. At a typical price per board foot you can then estimate the cost of a whole wall frame before you reach the checkout.

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