What a board foot is
A board foot is a volume: 144 cubic inches, the same as a board 1 in thick, 12 in wide and 12 in long. Hardwood dealers price by the board foot, so the formula is thickness (in) × width (in) × length (in) ÷ 144.
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Board foot calculator
List the parts you need, set a waste allowance, and get the board feet to buy and what it will cost. Works with rough lumber thicknesses like 4/4 and 8/4.
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Board feet are measured on rough (nominal) thickness: a 4/4 board counts as 1 in thick even though it mills to about 3/4 in. Enter the rough thickness you'll buy.
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Good to know
Rough versus finished thickness
Hardwood is sold in quarters of an inch of rough thickness: 4/4 is 1 in, 5/4 is 1-1/4 in, 8/4 is 2 in. You pay for the rough size, then lose about 1/4 in milling 4/4 flat to roughly 3/4 in. Dimensional lumber like a 2x4 is different: it's sold by the piece and is actually 1-1/2 × 3-1/2 in.
How much waste to allow
Allow about 20% for straight-grained work and 30 to 40% when you need matched grain and color, many short parts, or rough boards with defects. Buying a little extra from the same batch is cheaper than a trip back for a board that doesn't match.