The formula: thickness × width × length ÷ 12
Board feet per piece = (thickness in inches × width in inches × length in feet) ÷ 12. For a 2×6 board, 8 feet long: (2 × 6 × 8) ÷ 12 = 8 board feet per piece. Ten of those pieces at $5 per board foot: 80 total board feet × $5 = $400.
A different board size and quantity
For twenty 1×4 boards, each 10 feet long, at $3 per board foot: each piece is (1 × 4 × 10) ÷ 12 = 3.3333 board feet, for 66.6667 total board feet across all twenty pieces — costing 66.6667 × $3 = $200.
Why board feet exist as their own unit
Lumber comes in many different thickness/width/length combinations, but pricing needs one consistent unit to compare across them — a board foot (144 cubic inches, regardless of shape) lets a thin, wide board and a thick, narrow board be priced on the same fair basis if they contain the same total volume.
What board feet don't capture
Board feet measure raw material volume, not how many actual pieces a project needs to cover a given area or run a given length — that depends on the specific structure being built, which is why deck and fence calculators exist as their own separate tools rather than working purely from board feet.