Calculate board feet for lumber from the thickness, width, length, and quantity — the standard way lumber is priced.
board feet = (thickness_in × width_in × length_ft ÷ 12) × pieces
A board foot is 144 cubic inches of lumber — the standard unit for buying and pricing rough and dimensional lumber.
Lumber is sold by nominal size (a 2×4 is actually 1.5×3.5″). Board-foot pricing uses nominal dimensions.
A single 2 x 6 x 10 ft board is (2 x 6 x 10) ÷ 12 = 10 board feet. If you need 20 of them, that's 200 board feet before waste. Lumber yards price rough hardwood and some softwood by the board foot, so this is how you compare prices and estimate cost.
Note that board feet uses nominal dimensions. A '2x6' is actually 1.5 x 5.5 inches once surfaced, but board-foot pricing for dimensional lumber uses the nominal 2 x 6. For rough-sawn hardwood, measure the actual rough dimensions.
The board-foot formula is a definitional standard. Lumber yards price by board foot for rough lumber and by piece for dimensional studs.
Lumber is sized twice: the nominal dimension it is named and priced by, and the smaller actual dimension after drying and planing. A 4/4 board is sold as 1 inch but surfaces to about ¾ inch; a 2×4 actually measures 1½ by 3½ inches. Calculate board-foot volume on the nominal thickness for pricing, but design your joinery around the actual dimensions.
Rough hardwood thickness is specified in quarter-inches over four: 4/4 is 1 inch, 6/4 is 1½ inches, 8/4 is 2 inches. This is how lumberyards quote hardwood, and board-foot volume is figured on this rough thickness even though the board is planed thinner before you take it home.
Eight — (2 × 6 × 8) ÷ 12 = 8 board feet.
Multiply thickness in inches by width in inches by length in inches and divide by 144. If length is in feet, divide thickness times width times length by 12 instead.
Board feet measure volume (how hardwood and rough lumber are sold); linear feet measure length only (how dimensional softwood like studs is sold). You cannot convert between them without knowing the cross-section.
It is the quarter system for rough hardwood thickness: 4/4 is 1 inch, 8/4 is 2 inches. Board-foot volume is figured on this nominal rough thickness.