How to calculate board feet
A board foot is a unit of volume used to buy and sell lumber, especially hardwood. One board foot is a piece 1 inch thick, 12 inches wide and 12 inches long, which is 144 cubic inches.
Board feet = thickness (in) × width (in) × length (ft) ÷ 12
or = thickness (in) × width (in) × length (in) ÷ 144
Example: ten 4/4 boards, each 6 inches wide and 8 feet long, are 1 × 6 × 8 ÷ 12 = 4 board feet each, or 40 board feet in total. At $8 per board foot, that's $320.
Board feet per linear foot
Handy for dimensional lumber, using nominal sizes:
| Nominal size | Board feet per linear foot | Per 8 ft board |
|---|---|---|
| 1 × 4 | 0.33 | 2.67 |
| 1 × 6 | 0.50 | 4.00 |
| 2 × 4 | 0.67 | 5.33 |
| 2 × 6 | 1.00 | 8.00 |
| 2 × 8 | 1.33 | 10.67 |
| 2 × 10 | 1.67 | 13.33 |
| 2 × 12 | 2.00 | 16.00 |
| 4 × 4 | 1.33 | 10.67 |
Hardwood thickness in quarters
| Quarter size | Rough thickness | Typical surfaced thickness |
|---|---|---|
| 4/4 | 1 in | about 3/4–13/16 in |
| 5/4 | 1.25 in | about 1 in |
| 6/4 | 1.5 in | about 1 5/16 in |
| 8/4 | 2 in | about 1 3/4 in |
Surfaced sizes vary by mill, so check with your lumberyard. Board feet are normally counted on the rough thickness.
Buying tips
- Make a cut list first, then add up the board feet with extra for defects. It's easier to match grain when you have spare boards.
- Random-width hardwood is common. Measure each board's width and total them up, or average the widths across the stack.
- Compare prices per board foot, not per board, when boards differ in size.
Frequently asked questions
What is a board foot?
A board foot is a volume of wood equal to a piece 12 inches long, 12 inches wide and 1 inch thick, or 144 cubic inches. Hardwood lumber is usually priced by the board foot.
How do I calculate board feet?
Multiply thickness in inches by width in inches by length in feet, then divide by 12. A board 1 inch thick, 6 inches wide and 8 feet long is 1 × 6 × 8 ÷ 12 = 4 board feet.
Do I use nominal or actual size?
Lumberyards normally calculate board feet from the rough or nominal size, not the planed size. A 4/4 hardwood board is counted as 1 inch thick even though it may be planed to about 3/4 inch, and a 2×4 is counted as 2 × 4 even though it measures 1.5 × 3.5.
What does 4/4, 5/4 and 8/4 mean?
Hardwood thickness is described in quarters of an inch, measured rough. 4/4 (four quarter) is 1 inch, 5/4 is 1.25 inches, 6/4 is 1.5 inches and 8/4 is 2 inches.
How much extra lumber should I buy?
For furniture and woodworking projects, plan 20 to 30 percent extra to allow for defects, knots, grain matching and milling losses. For framing with dimensional lumber, 10 percent is usually enough.