How it works
A board foot is a volume of wood 1 inch thick, 12 inches wide and 12 inches long (144 cubic inches). Hardwood lumberyards price rough-sawn boards by the board foot.
Use the rough or nominal thickness that the yard sells by. Hardwoods are described in quarters: 4/4 is 1 inch, 5/4 is 1.25 inches, 8/4 is 2 inches. Softwood dimensional lumber like 2 × 4s is usually sold per piece or per linear foot, but board feet are still useful for comparing prices.
Rough hardwood loses thickness and width to planing and has knots, checks and sapwood you will cut around. Adding 20% to 30% to your project's finished board feet is a common buying rule.
Formula
Board feet = thickness (in) × width (in) × length (ft) ÷ 12. (Or thickness × width × length, all in inches, ÷ 144.)
Questions people ask
How many board feet in a 2 × 6 × 8?
Using nominal size: 2 × 6 × 8 ÷ 12 = 8 board feet.
Do I use actual or nominal dimensions?
Lumberyards use nominal or rough dimensions for board feet, so a 2 × 4 counts as 2 by 4 inches even though it measures 1.5 × 3.5.
How many board feet are in a cubic foot?
Twelve. A cubic foot is 1,728 cubic inches and a board foot is 144.