The formula
One board foot is 144 cubic inches: a piece 1 inch thick, 12 inches wide and 12 inches long. In practice you measure thickness in inches, width and length in inches, multiply and divide by 144.
For a 1 inch thick board 6 inches wide and 96 inches long, that is 1 × 6 × 96 ÷ 144 = 4 board feet. Metric stock is usually sold in cubic metres, where the same volume is about 0.0094 m³.
Thickness is nominal, width and length are actual
Hardwood is priced on nominal thickness — a board sold as 1 inch may finish at 3/4 inch after surfacing. You pay for the thickness the mill sold it as, not the thickness you measure.
Width and length, on the other hand, are measured as they are. Measure the narrowest point of the board, since a wany or tapered edge will be trimmed off.
Add for waste, always
A cutting list gives you the finished parts. The boards you buy must also cover saw kerfs, jointing, trimming, defects and mistakes.
Twenty percent is a reasonable allowance for a straightforward project in sound stock; thirty percent is more realistic when the boards have knots, checks or significant sapwood.
Lay out parts before you buy
Rough out your parts on paper over the board dimensions you expect to find. Long parts want long boards; small parts should be nested into the offcuts of the big ones.
Choosing boards in the yard with a cutting list in hand is how you avoid buying three boards when two would have done, and how you avoid coming home one part short.
