LUMBER VOLUME
How to Calculate Board Feet
Use the 144-cubic-inch definition, tally every board, then keep defects and milling loss separate from the purchased volume.
SHORT ANSWER
The working rule
For dimensions in inches, board feet = thickness × width × length ÷ 144. Use the dimensions and tally convention stated by the seller; then add a separate project allowance for defects, grain selection, surfacing, and joinery.
Start with the unit—not the nickname
A board foot is a volume of 144 cubic inches: the volume of a board 1 inch thick, 12 inches wide, and 12 inches long. A 1 × 6 × 96-inch board therefore contains 4 board feet. The same formula works for other dimensions as long as thickness, width, and length are all entered in inches.
Do not silently substitute a nominal label for a measured dimension. NIST distinguishes nominal softwood sizes from actual dressed sizes, and its retail hardwood rules define how board-foot sales are represented. Before tallying, note whether the yard prices rough, surfaced, random-width, or fixed-dimension stock and copy the dimensions shown on its ticket.
Build a board-by-board tally
For random-width hardwood, list each board or group boards with identical dimensions. Convert feet-and-inches lengths to inches, calculate each row, then sum the unrounded row values. If the supplier rounds width, length, or surface measure under a published tally rule, reproduce that rule rather than inventing a different rounding method.
Keep quantity and volume visible together. Ten boards at 4 board feet each are 40 board feet, but that total says nothing about whether their widths, lengths, color, and grain can produce the planned parts. A useful worksheet retains the original board dimensions so the arithmetic can be checked against the lumber-rack count.
Separate purchased volume from usable yield
Board feet measure volume, not clear cutting yield. End checks, wane, knots, sapwood restrictions, grain matching, cupping, and twist can remove usable regions. Surfacing also reduces thickness and width. Mark these as project constraints and add an allowance only after calculating the measured volume required by the cut list.
Use two totals: the net volume represented by the parts and the purchase target after a documented allowance. A blanket percentage is less useful than a note such as “extra for two matched door panels and one bowed board.” Revisit the allowance after selecting actual boards; wide clear stock may change the cutting plan more than the volume total does.
Run three checks before purchase
First, confirm every dimension uses the same unit. Second, spot-check one row by converting it to cubic inches and dividing by 144. Third, compare the tally with the seller’s invoice and ask which dimensions or rounding convention produced any difference. Preserve the answer with the project notes.
Finally, check the cut list against available lengths and widths. A project can have enough board feet and still lack a board long or wide enough for a critical part. Reserve continuous-grain or color-matched parts before allocating short components, then update the tally with the boards actually chosen.
A practical workflow
- Record actual or seller-tallied thickness, width, length, quantity, and units for every board.
- Calculate each row with thickness × width × length ÷ 144 and total the unrounded results.
- Compare net part volume with the board tally, then document allowances for defects, milling, and selection.
- Verify critical part lengths and widths against individual boards before committing to the purchase.