Board Foot Formula
Learn the US board-foot formula and work through practical examples.
Formula with inch cross-section and foot length
When thickness and width are in inches and length is in feet, board feet equals thickness × width × length × quantity ÷ 12. The divisor appears because a board foot contains twelve inch-feet of cross-sectional area. If all three dimensions are in inches, divide cubic inches by 144 instead.
Write units beside the numbers before multiplying. Unit labels catch the common error of entering an inch length into a feet field or dividing by 144 after the length has already been converted to feet.
Three worked examples
A 1 × 8 board 9 feet long is 6 BF. A 1.5 × 5.5 board 8 feet long is 5.5 BF. Twelve strips measuring 0.5 × 2 inches and 4 feet long total 4 BF. In each case, calculate the per-piece value first, then multiply by the whole-number quantity.
Fractions and decimals
Convert inch fractions carefully: 4/4 as a stated nominal thickness may be entered as 1, while a measured 13/16 inch is 0.8125. Do not write 13/16 as 0.1316. Retain the original fraction in an audit note when measurement precision matters.
Rounding order
Keep unrounded per-piece and row values until the final total unless a contract requires a different tally method. Rounding each of many small rows can create a material difference. BoardFoot Bench displays a readable result while its tally sums valid unrounded row totals.
Questions about this topic
Why divide by 12?
Length is already in feet while thickness and width are inches; the divisor reconciles the units.
When do I divide by 144?
When thickness, width and length are all expressed in inches.
Can quantity be a decimal?
Not for piece count in this model; use separate measurement logic for partial material.
Related BoardFoot Bench pages
Method and content reviewed 3 September 2026.