What a board foot is
A board foot is a unit of wood volume: 144 cubic inches, or a piece 1 inch thick, 12 inches wide and 12 inches long. Hardwood lumberyards price almost everything this way because boards come in random widths and lengths. A board foot lets them charge fairly for a 5-inch-wide board and a 9-inch-wide one without a separate price for each.
The formula
or: thickness (in) × width (in) × length (ft) ÷ 12
to buy = board feet × quantity × (1 + waste %)
cost = board feet to buy × price per board foot
Example: ten 4/4 oak boards
You need ten boards of 4/4 red oak, each 6 inches wide and 8 feet long, at $6.00 per board foot, with a 15% waste allowance.
| Step | Result |
|---|---|
| Per board: 1 × 6 × 96 ÷ 144 | 4 BF |
| Ten boards | 40 BF |
| Plus 15% waste | 46 BF |
| Cost: 46 × $6.00 | $276.00 |
| Cost per board, before waste | $24.00 |
Nominal vs actual lumber sizes
The size a board is sold as isn't the size you take home. Hardwood is sold by its rough thickness in quarter inches, then usually surfaced (planed) smooth. Softwood dimensional lumber is named by its size before drying and planing. Board feet are figured on the nominal (rough) numbers, so use those in the calculator, but plan your project around the actual dimensions.
| Sold as | Nominal | Typical actual (surfaced) |
|---|---|---|
| 4/4 hardwood | 1 in thick | 3/4 to 13/16 in |
| 5/4 hardwood | 1 1/4 in | 1 in to 1 1/16 in |
| 6/4 hardwood | 1 1/2 in | 1 1/4 to 1 5/16 in |
| 8/4 hardwood | 2 in | 1 3/4 in |
| 1x6 board | 1 × 6 in | 3/4 × 5 1/2 in |
| 2x4 | 2 × 4 in | 1 1/2 × 3 1/2 in |
| 2x6 | 2 × 6 in | 1 1/2 × 5 1/2 in |
| 4x4 | 4 × 4 in | 3 1/2 × 3 1/2 in |
Surfaced thicknesses vary a little between mills. If you buy rough lumber and plane it yourself, expect to lose about 3/16 to 1/4 inch of thickness on a 4/4 board.
Tips for buying hardwood
- Buy more than your cut list. 15% is a minimum for good stock; 25–30% is safer for lower grades or many small parts.
- Watch the grade. FAS (firsts and seconds) yields long, clear cuts. Common grades cost less but have more defects to cut around.
- Confirm how widths are measured. Rough boards with uneven edges are measured at their usable width; some yards round to the nearest inch.
- Check thickness for thin parts. If your project needs 1-inch finished stock, 4/4 won't do. Buy 5/4.
Frequently asked questions
How do you calculate a board foot?
Multiply thickness in inches by width in inches by length in inches, then divide by 144. If you measure length in feet, multiply thickness by width by length in feet and divide by 12. One board foot is a piece 1 inch thick, 12 inches wide and 12 inches long.
Do you use nominal or actual size for board feet?
Hardwood dealers calculate board feet on the rough (nominal) thickness, so a 4/4 board is figured at 1 inch even after it's surfaced to about 3/4 inch. Width and length are usually measured as the board actually is. Dimensional softwood like 2x4s is normally sold by the piece or linear foot, but when board feet are quoted, nominal sizes are used.
What does 4/4, 5/4 and 8/4 mean?
Hardwood thickness is sold in quarter inches of rough thickness. 4/4 (four quarter) is 1 inch rough, 5/4 is 1 1/4 inch, 6/4 is 1 1/2 inch and 8/4 is 2 inches. After surfacing both faces, 4/4 typically ends up around 13/16 to 3/4 inch.
How much waste should I add when buying lumber?
For rough hardwood, 15% to 30% is typical to cover defects, knots, checks at the ends, and cutting yield. Use the low end for clear, straight stock and simple cuts, and the high end for lower grades, figured wood, or projects with many small parts.
How many board feet are in a 2x4 that is 8 feet long?
Using nominal size, 2 × 4 × 96 ÷ 144 = 5.33 board feet. Using the actual 1.5 × 3.5 inch size it's 3.5 board feet. Lumberyards that quote board feet for dimensional lumber use the nominal figure.
Rates and figures last checked September 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.