What a board foot is
A board foot is a volume of wood: 1 inch thick, 12 inches wide and 12 inches long, or 144 cubic inches. Hardwood is usually sold by the board foot.
The formula
Board feet = thickness (in) × width (in) × length (ft) ÷ 12
- A board 1 in × 8 in × 6 ft: 1 × 8 × 6 ÷ 12 = 4 board feet.
- A 2 × 6 that is 8 ft long: 2 × 6 × 8 ÷ 12 = 8 board feet.
Multiply by the number of pieces for the total, and by the price per board foot for the cost: 8 board feet at $4.50 is $36.
Nominal and actual sizes
Construction lumber is named by its rough size before drying and planing: a "2 × 6" actually measures about 1½ × 5½ inches. Board feet for dimensional lumber are normally worked out from the nominal size, so use the name, not a tape measure.
The quarter system
Rough hardwood thickness is given in quarters of an inch: 4/4 ("four quarter") is 1 inch, 6/4 is 1½ inches and 8/4 is 2 inches. Use those numbers as the thickness in the formula.
The Board Feet Calculator works out the total for any number of pieces, with the cost.