One board foot is 144 cubic inches — a board 12 inches by 12 inches by 1 inch. A nominal 2×4 eight feet long is 5.33 board feet, so six of them are 32.00 board feet: 35.20 board feet with a 10% waste allowance, $158.40 at $4.50 per board foot.
The formula, in both forms
Board feet measure volume, not length or area:
- Length in inches: (thickness in inches × width in inches × length in inches) ÷ 144
- Length in feet: (thickness in inches × width in inches × length in feet) ÷ 12
The two forms are identical, not alternatives: length in feet times 12 is length in inches, and 144 ÷ 12 is 12. This calculator computes internally in inches, so the feet and inches entries always agree. Totals are built from the unrounded per-piece figure — multiply first, round only at display — which is why six 2×4×8 boards total exactly 32.00 board feet even though each piece displays as 5.33.
Nominal vs actual: the table that explains lumber sizes
| Nominal size | Actual size (in) |
|---|---|
| 1×4 | 0.75 × 3.5 |
| 1×6 | 0.75 × 5.5 |
| 2×4 | 1.5 × 3.5 |
| 2×6 | 1.5 × 5.5 |
| 2×8 | 1.5 × 7.25 |
| 2×10 | 1.5 × 9.25 |
| 2×12 | 1.5 × 11.25 |
| 4×4 | 3.5 × 3.5 |
Finished sizes are verified against two lumber-industry sources that agree on every row: the Western Wood Products Association (WWPA) Standard Sizes chart (wwpa.org) and Woodcraft's “Nominal Lumber Sizes Explained” guide (woodcraft.com). Three facts make the table make sense. Length is true to size — an 8 ft board is 96 in long; only thickness and width shrink. The reduction comes from drying plus surfacing the rough-sawn board. And the tally convention: dimensional (softwood) lumber board-foot counts and hardwood dealer tallies are conventionally figured on nominal/rough dimensions — which is why this calculator computes the buy figure from the dimensions as entered (nominal when a preset is used) and shows the actual-size figure for information only. Selecting a preset turns on that comparison line: same board, both measurements, side by side.
Hardwood thickness uses its own shorthand, the quarter system: 4/4 = 1 in, 5/4 = 1.25 in, 6/4 = 1.5 in, 8/4 = 2 in. The quick-pick chips beside the thickness field enter those values directly. A single 5/4 × 6 in board ten feet long, for reference, is (1.25 × 6 × 10) ÷ 12 = 6.25 board feet.
Measuring rough lumber, and choosing a waste factor
For rough boards with no preset, measure thickness and width with a tape at a few points and use the dimensions the yard will tally — rough dimensions, before surfacing. Length in inches suits short hardwood boards: one board at 1 in (4/4) × 6 in × 96 in is (1 × 6 × 96) ÷ 144 = 4.00 board feet. Enter 96 in the inches field and it governs over the feet field, exactly as the form states.
The waste factor is general practice, not a standard. Ten percent is the default here: straight, simple builds can run lower, while projects with grain matching, defects to cut around, or many short parts run 15–20% or more. The factor multiplies the volume you order; it does not guarantee part yield from any specific board layout, because yield depends on how your parts nest on the boards you actually receive.
Board foot vs linear foot vs square foot
Buyers confuse three different units. A linear foot is just length — one foot of a board, whatever its cross-section. A square foot is area — length × width, ignoring thickness. A board foot is volume — 144 cubic inches — which is why two boards with the same footprint but different thicknesses carry different board-foot counts and different prices. Lumber priced per board foot rewards you for checking the thickness; lumber priced per linear foot hides it.
Worked example
Six nominal 2×4 boards, each 8 ft long, at $4.50 per board foot with a 10% waste factor. The calculator loads with the 2×4 preset selected and exactly these figures — if your page shows different defaults, re-select the 2×4 preset to reproduce them:
- Per piece: (2 × 4 × 8) ÷ 12 = 5.33 board feet (exact 5.3333…)
- Total: 5.3333… × 6 = 32.00 board feet
- Order quantity with waste: 32.00 × 1.10 = 35.20 board feet
- Total cost: 35.20 × $4.50 = $158.40; cost before waste: 32.00 × $4.50 = $144.00
- Actual-size comparison: (1.5 × 3.5 × 8) ÷ 12 = 3.50 board feet per piece (21.00 for six) — information only, not the buy figure
Leave the price blank and the cost rows hide entirely rather than showing a meaningless $0.00 — the volume figures are the answer the yard needs first.