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A board foot is 144 cubic inches of wood: a board 1 in thick, 12 in wide and 1 ft long. A board's volume in board feet is its thickness in inches times its width in inches times its length in feet, divided by 12. A 4/4 board 6 in wide and 8 ft long is 4 board feet.
Rough hardwood is sold by its thickness in quarters of an inch: 4/4 is 1 in, 5/4 is 1 1/4 in, 8/4 is 2 in. Type the quarters as they're written, or the thickness in inches. Board feet are counted on that rough size even after surfacing: a 4/4 board planed on both faces (S2S) measures 13/16 in, but it's tallied as 1 in.
Construction lumber is counted by its nominal size, not its actual size. A 2×4 measures 1 1/2 × 3 1/2 in, but an 8 ft 2×4 is 2 × 4 × 8 / 12 = 5.33 board feet. Enter the nominal size to match a lumber yard's tally.
Hardwood usually comes in random widths and lengths, so each board is measured on its own. Enter width in inches, and length in feet or, for short pieces, in inches. Fractions work: 7 1/2, 7-1/2 or 7.5.
Each board can carry its own price per board foot, since species and grade change the price. A board without a price still counts toward the board feet; the cost line says how many are unpriced. Sales tax and delivery aren't included.
Some yards round each board to the nearest whole board foot, a half rounding up, before multiplying by the quantity. Choose Nearest whole BF to match their tally. Exact keeps the fractions.
One board foot is 0.00236 cubic meters, and a cubic meter is about 424 board feet. The total shows both. Plywood and other sheet goods are sold by the sheet, so board feet don't apply to them.