The formula: board feet
One board foot is a piece 1 in. thick, 12 in. wide and 12 in. long, which is 144 cubic inches. Lumber is figured on its nominal size, the size it is named by, not the smaller size it is dressed to.
- Board feet per piece = thickness (in.) × width (in.) × length (ft) ÷ 12.
- Board feet per line = board feet per piece × number of pieces.
- MBF = board feet ÷ 1,000, and cost = MBF × price per MBF.
The 12 comes from mixing units: inches times inches times feet is 1/12 of a board foot for each unit, because a board foot is 1 in. × 12 in. × 1 ft.
Worked example 1: one size
Forty 2x10s, each 16 ft long, at $950 per MBF. These are invented study numbers.
- One piece = 2 × 10 × 16 ÷ 12 = 320 ÷ 12 = 26.67 board feet.
- Forty pieces = 320 × 40 ÷ 12 = 12,800 ÷ 12 = 1,066.67 board feet.
- MBF = 1,066.67 ÷ 1,000 = 1.0667. Cost = 12,800 ÷ 12,000 × 950 = $1,013.33.
Check per piece: 26 2/3 board feet × 40 = 1,066 2/3, and 1,066 2/3 × 0.95 = 1,013.33.
Worked example 2: a mixed list with waste
Twenty-four 2x4s at 8 ft, twelve 2x12s at 20 ft and six 4x4s at 10 ft, with 5% waste, at $800 per MBF.
- 2x4s: 2 × 4 × 8 × 24 ÷ 12 = 1,536 ÷ 12 = 128.
- 2x12s: 2 × 12 × 20 × 12 ÷ 12 = 5,760 ÷ 12 = 480.
- 4x4s: 4 × 4 × 10 × 6 ÷ 12 = 960 ÷ 12 = 80.
- Total = 128 + 480 + 80 = 688 board feet. With 5% waste: 688 × 1.05 = 722.4 board feet.
- Cost = 0.7224 × 800 = $577.92.
Check per piece: a 2x4 at 8 ft is 5 1/3 board feet and 5 1/3 × 24 = 128; a 2x12 at 20 ft is 40 and 40 × 12 = 480; a 4x4 at 10 ft is 13 1/3 and 13 1/3 × 6 = 80. Then 688 × 0.8 = $550.40 before waste, and 550.40 × 1.05 = $577.92. These are original arithmetic examples, not exam questions.
Board feet questions
How do you calculate board feet?
Multiply the nominal thickness in inches by the nominal width in inches by the length in feet, and divide by 12. One 2x10 that is 16 ft long is 2 × 10 × 16 ÷ 12 = 26.67 board feet, and forty of them are 2 × 10 × 16 × 40 ÷ 12 = 1,066.67 board feet.
Should I use the nominal or the actual size?
Nominal. Board feet are figured on the named size, so a 2x10 counts as 2 by 10 even though the dressed piece is smaller. Using the dressed size undercounts the board feet.
How do I price lumber by the MBF?
MBF means a thousand board feet. Divide the board feet by 1,000 and multiply by the price per MBF. In the worked example, 1,066.67 board feet is 1.0667 MBF, and at $950 per MBF that is 12,800 × 950 ÷ 12,000 = $1,013.33.
Where is this on the NASCLA exam?
Wood is one of the twelve subject areas on the NASCLA outline, with 5 of the 115 scored questions. The Bid Bond course works board feet in its Wood lesson.
See the NASCLA exam outline for all twelve subject areas and their question counts.