The formula
From Carpentry and Building Construction, board-foot takeoff:
BF = T × W × L ÷ 12
- T = thickness in inches (nominal).
- W = width in inches (nominal).
- L = length in feet.
Worked takeoffs
- Ten 2x10s, 16 ft: 2 × 10 × 16 ÷ 12 = 26.667 BF each; × 10 = 266.67 BF.
- Sixty 2x4 studs, 8 ft: 2 × 4 × 8 ÷ 12 = 5.333 BF each; × 60 = 320 BF.
- One 4x12 beam, 20 ft: 4 × 12 × 20 ÷ 12 = 80 BF.
- 240 linear feet of 2x6 plate: 2 × 6 × 240 ÷ 12 = 240 BF.
For identical pieces, work out one piece, then multiply by the count.
Shortcut: BF per linear foot
T × W ÷ 12 is the board feet in one linear foot. A nominal 2x6 holds exactly 1 BF per linear foot (2 × 6 ÷ 12 = 1), so linear feet of 2x6 equal board feet. A 1x6 holds half a board foot per foot; counting 2x6 at that rate gives 120 BF instead of 240 BF.
The three traps
- Actual dimensions. Board feet use nominal sizes. Plugging in 1½ × 9¼ for a 2x10 gives 185 BF instead of 266.67; 1½ × 3½ for the studs gives 210 instead of 320; 3½ × 11¼ for the 4x12 gives 65.63 instead of 80; 1½ × 5½ for the plate gives 165 instead of 240.
- Dividing by 144. That divisor is for length in inches. With length in feet it gives 22.22, 26.67 or 6.67, all far too small.
- Leaving out ÷ 12. That gives 3,200, 3,840, 960 or 2,880, twelve times too big.