Lumber is bought and sold by volume, and the board foot is the trade’s unit of it: a piece 1 inch thick by 12 inches wide by 1 foot long. The working formula is BF = thickness (in) × width (in) × length (ft) ÷ 12, and by firm trade convention the thickness and width are always the NOMINAL dimensions — a 2x6 counts as 2 by 6 even though it dresses to 1.5 by 5.5 inches. An 8-ft 2x6 is therefore 2 × 6 × 8/12 = 8 board feet, and ten of them make an 80 BF line item.
Hardwood dealers speak the same unit with a different thickness dialect: quarters notation, where 4/4 is 1 inch, 5/4 is 1.25, 6/4 is 1.5, and 8/4 is 2 — so a 5/4 × 6 board 10 ft long carries 1.25 × 6 × 10/12 = 6.25 BF. Pricing rides directly on the unit as dollars per board foot: the 80 BF framing order at $0.85/BF costs $68.00, and a takeoff list totals its board feet and costs line by line.