What a board foot is
A board foot is a piece of wood 1 inch thick, 12 inches wide and 12 inches long. That's 144 cubic inches, or 1/12 of a cubic foot. Lumberyards use it to price hardwood and rough-sawn lumber, because boards come in so many widths and lengths that pricing per piece doesn't work.
The board foot formula
There are two versions. Use whichever matches how you measured length:
- Length in feet: board feet = thickness (in) × width (in) × length (ft) ÷ 12
- Length in inches: board feet = thickness (in) × width (in) × length (in) ÷ 144
Multiply the result by the number of boards to get the total for a stack.
Worked examples
| Board | Math | Board feet |
|---|---|---|
| 1 × 6, 8 ft | 1 × 6 × 8 ÷ 12 | 4 |
| 2 × 4, 8 ft | 2 × 4 × 8 ÷ 12 | 5.33 |
| 2 × 10, 12 ft | 2 × 10 × 12 ÷ 12 | 20 |
| 8/4 walnut, 6 in wide, 10 ft | 2 × 6 × 10 ÷ 12 | 10 |
So 20 pieces of 2 × 4 × 8 ft are 20 × 5.33 = about 107 board feet. The board foot calculator handles mixed sizes and quantities in one go.
Nominal vs. actual size
Softwood dimensional lumber is named by its nominal size. A 2 × 4 actually measures about 1½ × 3½ inches after drying and planing. By convention, board feet for dimensional lumber are calculated from the nominal size, so a 2 × 4 × 8 is 5.33 board feet even though the wood in it is less.
Hardwood is different. It's usually sold rough-sawn and measured by its actual thickness and width, with random widths and lengths. When you compare prices, check which convention the supplier uses.
The quarter system for hardwood
Hardwood thickness is given in quarters of an inch:
| Name | Rough thickness |
|---|---|
| 4/4 (“four quarter”) | 1 in |
| 5/4 | 1¼ in |
| 6/4 | 1½ in |
| 8/4 | 2 in |
| 12/4 | 3 in |
Use the rough thickness in the formula. A board planed to ¾ in from 4/4 stock is still sold as 4/4.
Pricing by the board foot
Multiply total board feet by the price per board foot. If walnut costs $12 per board foot, the 10 board-foot board above costs $120. Add a waste allowance before you buy: 10–15% for straightforward projects, and 20–30% for furniture where you'll cut around knots, defects and color changes.
If a store lists dimensional lumber by the piece, divide the piece price by its board feet to compare it with a lumberyard's per-board-foot rate.
Board feet vs. linear feet
Linear feet measure only length, regardless of width or thickness. Trim, molding and decking are often sold that way. Board feet account for all three dimensions, which is why they're used for rough stock. For framing, it's often simpler to count pieces: the stud calculator gives the number of studs for a wall.