Board foot

The formula

Board feet = thickness × width × length, all in inches, ÷ 144.

If you measure the length in feet, use thickness (in) × width (in) × length (ft) ÷ 12 instead. Both give the same answer.

A board 1 inch thick, 6 inches wide and 8 feet (96 inches) long is 1 × 6 × 96 ÷ 144 = 4 board feet.

Which size goes into the formula

The answer changes depending on whether you use the board’s name size or its measured size, so check which one a seller uses.

• Softwood (pine, fir, spruce and other building lumber). The American Softwood Lumber Standard figures board feet from the nominal size. An 8 foot 2 × 4 is 2 × 4 × 8 ÷ 12 = 5⅓ board feet, even though it really measures about 1½ × 3½ inches.
• Hardwood (oak, maple, walnut and other furniture woods). Thickness is named in quarter inches of rough, unplaned wood: 4/4 (“four quarter”) is 1 inch and 8/4 is 2 inches. The USDA Wood Handbook lists 4/4 as planing to 13/16 inch. If you buy planed boards, ask whether the board feet were worked out from 1 inch or from 13/16 inch, as the two differ by almost 19%.

Board feet or linear feet

Some yards price lumber per board foot and some per linear foot, which counts length only. A 1 × 6 that is 8 ft long is 4 board feet but 8 linear feet, so ask which one a price means before you compare.