A nominal 2 × 6 is not 2 inches by 6 inches. Dressed (surfaced) dimensional lumber measures about 1.5 × 5.5 in; rough lumber may measure full nominal size. The difference matters twice: once for board-foot billing, and once for any physical fit — a joist bay, a shelf span, a deck row count.
Billing convention versus physical volume
Board feet = thickness (in) × width (in) × length (ft) ÷ 12 × pieces. On a nominal billing basis, 10 pieces of 2 × 6 × 8 ft are 80 board feet. On an actual-dimension basis, entering 1.5 × 5.5 in instead gives 55 board feet. Both figures are “correct” — for different conventions. The error is applying a price quoted on one basis to a quantity computed on the other. The Board Foot Calculator therefore labels the basis in the result and never substitutes dimensions silently.
Deck boards show the same trap
A nominal 6-inch deck board is commonly 5.5 in of face width, and deck rows are counted from face width plus gap. Using nominal width under-counts rows across a wide deck. The Deck Board Calculator asks for actual face width for exactly this reason. Whenever a dimension is quoted, ask: nominal, actual, or module (including joint, as for brick and block)? The answer changes the count.