How to Calculate Board Feet: Formula and Worked Examples
How to calculate board feet from thickness, width and length, with worked examples for a single board and a full lumber order.
To calculate board feet, multiply thickness in inches by width in inches by length in feet, then divide by 12. A board 2 inches thick, 6 inches wide and 8 feet long is 2 × 6 × 8 = 96, and 96 ÷ 12 gives 8 board feet.
Why the formula divides by 12
A board foot is 144 cubic inches: a board 1 inch thick, 12 inches wide and 1 foot long. Multiplying thickness by width by length in feet gives you 12 times the number of board feet, because the width is in inches while the length is already in feet. Dividing by 12 corrects for that. If you work with all three measurements in inches, divide by 144 instead.
Use the nominal size
Lumber is counted on the size printed on the tag, not the size you measure with a tape. A 2x6 is about 1.5 by 5.5 inches after drying and planing, yet it is figured as 2 by 6. Hardwood sold rough is different: it is measured in quarters of an inch, so 4/4 means 1 inch thick and 8/4 means 2 inches.
Worked example: a whole order
| Item | Math | Board feet |
|---|---|---|
| Ten 2x6 boards, 8 ft | 2 × 6 × 8 ÷ 12 = 8 each | 80 |
| Six 4x4 posts, 10 ft | 4 × 4 × 10 ÷ 12 = 13.33 each | 80 |
| Twenty 1x6 boards, 12 ft | 1 × 6 × 12 ÷ 12 = 6 each | 120 |
| Total | 280 |
Add 10% for waste and the order comes to 308 board feet. At $4.50 per board foot that is $1,386. The Board Foot Calculator does each line for you and adds the waste and cost. If you are framing a wall, the Wall Framing Calculator counts the studs and plates first, and you can turn that piece list into board feet here.