A board foot is a unit of volume: 144 cubic inches, the same as a piece of wood 12 inches long, 12 inches wide and 1 inch thick. Hardwood dealers in the US usually price lumber per board foot, and lumberyards use the same unit for softwood sold in quantity. If you can work out board feet quickly, you can compare prices between boards of different sizes and estimate what a project will cost before you drive to the yard.
The board foot formula
With thickness and width in inches and length in feet:
board feet = thickness (in) × width (in) × length (ft) ÷ 12
With everything in inches:
board feet = thickness (in) × width (in) × length (in) ÷ 144
Both give the same result. A board 1 inch thick, 6 inches wide and 8 feet long is 1 × 6 × 8 ÷ 12 = 4 board feet; in inches, 1 × 6 × 96 ÷ 144 = 4. For a stack of identical boards, multiply by the count at the end. The board foot calculator does the arithmetic for a whole list.
Quarters: what 4/4, 5/4 and 8/4 mean
Hardwood thickness is stated in quarters of an inch, measured rough: as sawn and dried, before planing. Four quarter (4/4) is 1 inch, 5/4 is 1 1/4 inches and 8/4 is 2 inches. The quarter system has nothing to do with quartersawn lumber, which describes how a board was cut from the log, not how thick it is.
| Quarter | Rough thickness | Typical after surfacing two sides (S2S) |
|---|---|---|
| 4/4 | 1 in (25.4 mm) | about 13/16 in (21 mm) |
| 5/4 | 1 1/4 in (31.8 mm) | about 1 1/16 in (27 mm) |
| 6/4 | 1 1/2 in (38.1 mm) | about 1 5/16 in (33 mm) |
| 8/4 | 2 in (50.8 mm) | about 1 3/4 in (44 mm) |
| 12/4 | 3 in (76.2 mm) | about 2 3/4 in (70 mm) |
Surfaced thicknesses vary by yard and by how flat the boards were, so check with calipers if a joint depends on it. If your project calls for 3/4-inch parts, you are buying 4/4 stock.
Nominal versus actual size
Most hardwood yards calculate board feet from the rough thickness, even when a board has been planed. A 4/4 board surfaced to 13/16 inch is still tallied as 1 inch thick. Width and length are measured on the board itself, since hardwood comes in random widths and lengths, but yards round those measurements differently, so ask how they tally if you are comparing prices closely.
Softwood dimensional lumber uses nominal names that are larger than the real sizes. A 2×4 is 1 1/2 × 3 1/2 inches, a 1×6 is 3/4 × 5 1/2 and a 4×4 is 3 1/2 × 3 1/2. Where softwood is sold by the board foot, the nominal sizes are normally used, so an 8-foot 2×4 counts as 5.33 board feet even though its actual volume is 3.5. At home centers dimensional lumber is usually sold by the piece, and the useful question becomes how many boards you need, which a lumber cut list answers directly.
Worked examples
| Board | Calculation | Board feet |
|---|---|---|
| 4/4 × 6 in × 8 ft | 1 × 6 × 8 ÷ 12 | 4.00 |
| 5/4 × 5 in × 6 ft | 1.25 × 5 × 6 ÷ 12 | 3.13 |
| 6/4 × 7 in × 8 ft | 1.5 × 7 × 8 ÷ 12 | 7.00 |
| 8/4 × 8 in × 10 ft | 2 × 8 × 10 ÷ 12 | 13.33 |
| 12/4 × 3 in × 30 in (leg blank) | 3 × 3 × 30 ÷ 144 | 1.88 |
| 2×4 × 8 ft (nominal) | 2 × 4 × 8 ÷ 12 | 5.33 |
| 4×4 × 8 ft (nominal) | 4 × 4 × 8 ÷ 12 | 10.67 |
To price a board, multiply its board feet by the price per board foot. At a hypothetical $9 per board foot, the 8/4 board in the table costs 13.33 × 9, or about $120. Sawmills quote per thousand board feet (MBF) instead — $1,200 per MBF is $1.20 per board foot; the board foot price calculator converts between per BF, per MBF, per linear foot and per piece.
Logs are measured differently. Before a log is sawn, its lumber yield is estimated from the small-end diameter and length with a log rule — Doyle, Scribner or International ¼-inch — and the three can disagree by 25% or more on the same log. The log board feet calculator compares them side by side.
From parts list to purchase
Board feet measure volume, but you buy whole boards, and boards have defects, checked ends and widths that don't match your parts. The figure shows a narrow wall shelf made from 4/4 × 6-inch boards: two 47-inch sides, a 30-inch top and four 28-inch shelves, cut with a 1/8-inch kerf. The parts add up to 236 inches of 6-inch stock, which is 1 × 6 × 236 ÷ 144 = 9.83 board feet. Cutting them takes three 8-foot boards, or 12 board feet, about 22% more than the parts themselves.
That 22% assumes clear boards with square ends. Rough lumber often needs an inch or two cut off each end to remove checks, and knots or splits have to be cut around, so the real figure is usually higher.
How much extra to buy
A common rule of thumb for hardwood projects is to buy 20 to 40% more than the board footage of the finished parts. Aim near the low end for clear, high-grade stock and simple parts. Aim near the high end for lower grades with more defects, for many short or narrow parts, and when you need to match color or figure across a set of parts such as drawer fronts or a tabletop glue-up. When a species is hard to find, a little extra costs less than a second trip and a board that doesn't match.
Board feet in metric
One board foot is 144 cubic inches, or about 2,360 cubic centimeters: roughly 0.00236 cubic meters. One cubic meter is about 423.8 board feet. The 8/4 × 8-inch × 10-foot board from the table works out to 13.33 × 0.00236, or about 0.0315 cubic meters.
Where timber is sold in metric sizes, multiply thickness and width in millimeters by length in meters and divide by 1,000,000 to get cubic meters. A 25 × 150 mm board 2.4 m long is 25 × 150 × 2.4 ÷ 1,000,000 = 0.009 cubic meters, or about 3.8 board feet.
Linear feet versus board feet
A linear foot is 12 inches of length, whatever the board's thickness or width. Mouldings, trim and surfaced boards at home centers are often priced this way. To compare a linear-foot price with a board-foot price, work out the board feet in one linear foot: thickness × width ÷ 12. A 4/4 × 6-inch board holds 0.5 board feet per linear foot, so $4.50 per linear foot is the same as $9 per board foot. A nominal 2×4 holds 0.67 board feet per linear foot.
For a full project, the board foot calculator totals volume and cost, and how to make a cut list covers getting the parts list right first. A 2×4 workbench is a good example of a project where counting boards matters more than counting board feet.