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A board foot is 144 cubic inches of lumber — one inch thick, one foot square. The formula is thickness × width × length in inches, divided by 144.
| Board | Per foot of length | 8 ft piece | 12 ft piece |
|---|---|---|---|
| 1 × 4 | 0.33 bd ft | 2.67 bd ft | 4.00 bd ft |
| 1 × 6 | 0.50 bd ft | 4.00 bd ft | 6.00 bd ft |
| 1 × 8 | 0.67 bd ft | 5.33 bd ft | 8.00 bd ft |
| 1 × 12 | 1.00 bd ft | 8.00 bd ft | 12.00 bd ft |
| 2 × 4 | 0.67 bd ft | 5.33 bd ft | 8.00 bd ft |
| 2 × 6 | 1.00 bd ft | 8.00 bd ft | 12.00 bd ft |
| 2 × 8 | 1.33 bd ft | 10.67 bd ft | 16.00 bd ft |
| 2 × 10 | 1.67 bd ft | 13.33 bd ft | 20.00 bd ft |
| 2 × 12 | 2.00 bd ft | 16.00 bd ft | 24.00 bd ft |
| 4 × 4 | 1.33 bd ft | 10.67 bd ft | 16.00 bd ft |
Every figure is thickness × width ÷ 12 per foot of length. Note that these use the nominal size — a 2 × 4 is billed as 2 × 4 even though it measures 1.5 × 3.5 after planing.
The definition. A board foot is the volume of a board one inch thick, one foot wide and one foot long — 144 cubic inches. It is a volume unit, which is why it works across boards of wildly different shapes.
The formula. Thickness in inches × width in inches × length in inches ÷ 144. If your length is in feet, the shortcut is thickness × width × length in feet ÷ 12, which is the same thing.
Nominal versus actual. A 2 × 4 measures 1.5 × 3.5 inches after it is planed, but it is bought, sold and billed as a 2 × 4. Board footage for construction lumber uses the nominal size. Hardwood is different — it is sold by the actual measured size, in quarters: 4/4 is one inch, 8/4 is two.
Board feet are not linear feet. Linear feet count length only, which is why trim and moulding are sold that way. Board feet count volume, which is why hardwood and rough lumber are. A 12 ft 2 × 12 is 12 linear feet and 24 board feet.
A board foot is 1/12 of a cubic foot. Twelve board feet make a cubic foot, and 324 board feet make a cubic yard. Useful when a quote arrives in the wrong unit.
144 cubic inches of lumber — a board one inch thick, one foot wide and one foot long. It measures volume, not length, which is how one unit can price boards of any shape.
Thickness in inches × width in inches × length in inches, divided by 144. With the length in feet instead, divide by 12. A 2 × 6 that is 10 ft long is 2 × 6 × 10 ÷ 12 = 10 board feet.
0.67 board feet per foot of length, so 5.33 in an 8 ft stick and 8 in a 12 ft one. That uses the nominal 2 × 4 size, which is how it is billed even though it measures 1.5 × 3.5.
Linear feet measure length alone; board feet measure volume. A 12 ft 2 × 12 and a 12 ft 2 × 4 are both 12 linear feet, but 24 board feet and 8 board feet respectively.
Quarters of an inch of thickness, measured rough. 4/4 is one inch, 5/4 is an inch and a quarter, 8/4 is two inches. Hardwood is sold by actual rough size, unlike construction softwood.