How this works
A board foot (BF) is the US lumber industry's standard volume unit: one board foot equals a piece of wood 1 inch thick, 12 inches wide and 12 inches long, or any combination with the same total volume of 144 cubic inches. The formula is BF = (thickness_in × width_in × length_in) / 144, or equivalently (thickness_in × width_in × length_ft) / 12 when length is in feet. A standard 2×4×8 — two inches thick, four inches wide, eight feet long — is 2 × 4 × 8 / 12 = 5.33 BF per piece. Sawmills, lumberyards and rough-sawn hardwood dealers price by the BF; dimensional softwood lumber at big-box stores is usually priced per piece, but the BF math still tells you whether you're paying a fair per-volume price.
A quirk that catches everyone the first time: nominal lumber dimensions don't match actual dimensions for surfaced softwood. A "2×4" is actually 1.5 inches by 3.5 inches after planing; a "1×6" is 0.75 by 5.5. The board-foot calculation in the calculator above uses whatever dimensions you enter — for buying surfaced lumber by the piece, enter the nominal dimensions (2 × 4 × 8) and you'll get the price-equivalent BF the seller charges by. For rough-sawn hardwood (which is sold by actual dimensions), enter the actual measured dimensions. The two won't match for the same physical piece, and that's the source of most "why does the math not add up" conversations at the lumberyard.
Three practical points. (1) Always over-order by 10-15% for cuts, mistakes and defects in the stock you receive (knots, splits, twist). The waste allowance in the calculator handles this; round up to the nearest whole piece at the yard. (2) For long pieces (16 feet or more), check available lengths before designing — many yards stock 2× material only up to 16', and 20'/24' lengths are special-order with a price premium. Cutting two 8-footers from a 16-footer is usually cheaper than buying one 20-footer. (3) Outside the US, lumber is sold by the cubic metre or the linear metre depending on country and product. Conversion: 1 BF ≈ 0.00236 m³, or about 424 BF per cubic metre. Dimensional metric lumber is typically a clean 38×89 mm (the metric "2×4") rather than imperial 2×4 actual sizes.
The formula
Thickness and width are in inches; length can be in inches or feet (the calculator handles both). For surfaced softwood enter the nominal size (a 2×4 is "2 by 4"); for rough-sawn hardwood enter the actual measured dimensions. The waste allowance is your over-order margin: 10% is standard for clean cuts, 15% for mixed-defect stock or projects with lots of short pieces.
Example calculation
- 5 pieces of 2×4×8 (two-by-four, eight feet long).
- BF per piece = (2 × 4 × 8) / 12 = 5.333 BF.
- Total = 5.333 × 5 = 26.67 BF. With 10% waste = 29.33 BF — round to 6 pieces.
Frequently asked questions
Should I use nominal or actual dimensions?
Match what the seller is charging by. For surfaced softwood (the 2×4s, 2×6s, 1×8s in big-box stores), enter the nominal dimensions — that's how the seller computes price-per-piece equivalent BF. For rough-sawn hardwood from a specialty dealer, enter the actual measured dimensions because that's the dealer's billing basis. The two answers will differ by ~25-40% for the same physical piece (a "2×4" surfaced is really 1.5"×3.5", which is 65.6% of nominal volume), and that gap is exactly the difference between "what the saw cut" and "what you took home". Don't mix the two — pick one convention per project and stick to it.
How much waste should I add?
10% is the default for clean rip-and-crosscut work on dimensional lumber where you have control over the layout. Push to 15% for projects with lots of short pieces (where offcuts pile up faster than you can reuse them) or for hardwood stock that comes with knot-and-defect grading — even "select grade" hardwood usually has 5-10% genuine unusable area. Drop to 5% only for very simple projects (a single straight cut per piece) where you've also accepted that one bad board means a return trip. As with most material calculators, running short mid-project costs more than buying one extra board.
How does this convert to metric?
One board foot equals approximately 0.00236 cubic metres (or 2.36 litres). Inverting: 1 cubic metre is about 424 BF. Outside the US, lumber is most often priced by the cubic metre or the linear metre depending on the country and product class — UK and EU softwood is typically m³, metric hardwood is sometimes m³ and sometimes m, and Japan uses both 立米 (cubic metre) and the traditional 才 (sai, ~0.00334 m³) for traditional carpentry. If you're sourcing internationally, ask the supplier which unit they're billing by before you compare prices; a price-per-BF that looks high may translate to a competitive price-per-m³ once converted, or vice versa.