The formula
Board feet = thickness (in) × width (in) × length (in) × quantity ÷ 144
Leave the length in feet and divide by 12 instead of 144. That is the same volume, because 144 is 12 inches times 12.
Board feet = thickness (in) × width (in) × length (ft) × quantity ÷ 12
One board foot is 144 cubic inches, about 0.00236 cubic metres. Four board feet is about 0.00944 cubic metres. CutCalc uses that conversion, 144 × 25.4³ cubic millimetres, so the two units are one volume written two ways.
Thickness, width, and length all have to be in the units the formula names. A suffix such as mm or ft on the board foot calculator converts the field before this division.
Nominal size and the stick you measure
Softwood sold as a 2×4 is named for the size before dressing. Under the American Softwood Lumber Standard, a dry dressed 2×4 is about 1½ by 3½ inches. Canadian dimension lumber of that class is sold to a millimetre size, 38 by 89, which is the standard itself and not a conversion of the inch fraction. Those dressed sizes are listed on the nominal and actual lumber sizes page.
Hardwood is often sold rough, by the quarter of the sawn thickness. A 4/4 board is counted as 1 inch thick even when a corner is thin. After it is surfaced two sides, the same board may measure about 13/16 inch. If the invoice used the rough inch, you already paid for that inch. If you are counting a board you just measured, use the caliper.
A yard that sells 2×4s by the piece is not selling board feet. The board-foot number is still the volume of wood in the stick. Use the dressed size for that volume. Use the nominal size only when someone is billing the name.
Worked examples
One board, 1 inch thick, 6 inches wide, 8 feet long. The length in inches is 8 × 12 = 96. Then 1 × 6 × 96 = 576 cubic inches, and 576 ÷ 144 = 4 board feet. With the length left in feet: 1 × 6 × 8 ÷ 12 = 4. Type thickness 1, width 6, and length 8 feet on the calculator and it returns 4.
Three rough 4/4 boards, 8 inches wide and 10 feet long. Count each as 1 inch thick, because 4/4 means that sawn thickness. 1 × 8 × 10 ÷ 12 = 6.667 board feet each. Times 3 is 20 board feet.
The same 1 × 6 after it is surfaced to 13/16 inch (0.8125), if you are measuring the dressed board rather than the invoice: 0.8125 × 6 × 8 ÷ 12 = 3.25 board feet. The face did not change. The thickness did.
An eight-foot 2×4, two ways. The name: 2 × 4 × 8 ÷ 12 = 5.333 board feet. The dressed stick: 1.5 × 3.5 × 8 ÷ 12 = 3.5 board feet. The gap is the wood the planer removed, not a second formula.
| Board | Entered as | Board feet |
|---|---|---|
| 1 × 6, 8 ft, one | 1 × 6 × 96 ÷ 144 | 4 |
| Three rough 4/4, 8 in × 10 ft | 1 × 8 × 120 ÷ 144, × 3 | 20 |
| That 1 × 6 surfaced to 13/16 | 0.8125 × 6 × 96 ÷ 144 | 3.25 |
| Dressed 2×4, 8 ft | 1.5 × 3.5 × 96 ÷ 144 | 3.5 |
| 2×4 by the name, 8 ft | 2 × 4 × 96 ÷ 144 | 5.333 |
| The 4 board feet, plus 15% waste | 4 × 1.15 | 4.6 |
Waste allowance
Waste multiplies the volume you need. It does not stretch the board, and it does not add a percent to one edge. Fifteen percent on the 4 board-foot example is 4 × 1.15 = 4.6 board feet to order.
Ten to twenty percent is a common furniture allowance for knots you will not use, a sapwood edge, and the offcuts between parts. It is a planning number. It is not a measurement of the pile at the yard, and it will not tell you that a particular board is clear. The calculator does the same multiplication: volume × (1 + waste ÷ 100).
What a board foot will not tell you
- It is not the square footage of the face, except when the board is exactly 1 inch thick. At 2 inches thick, the board feet are double the face in square feet.
- It does not grade the board. Wane, checks, and knots are still inside the volume.
- It does not say the board is flat, and it does not include saw kerf between the parts you will cut. Kerf is a separate count of the cuts.
Questions
Do I enter the name of a 2×4, or the size I measure?
Enter the size that matches how the board is being counted. A dry dressed 2×4 is about 1½ by 3½ inches, which is 3.5 board feet at eight feet. The name 2 by 4 gives 5⅓ board feet for the same length. Dimension lumber is usually sold by the piece, so the board-foot figure is the volume of wood, not the price tag. Use the nominal size only when a seller is billing that name.
Does planing change the board footage I already paid for?
Planing removes wood, so a measured board gets smaller. A rough 4/4 board is counted as 1 inch thick. After it is surfaced two sides it may measure about 13/16 inch. If the dealer billed the rough inch, that invoice does not shrink. If you are pricing a board you just measured, use the thickness on the tape.
How much waste should I add?
Waste is a buying allowance, not a longer board. Ten to twenty percent is a common furniture allowance for defects, sapwood, and offcuts. Fifteen percent on 4 board feet is 4.6 board feet to order. It is not a measurement of your pile, and it does not grade the boards.
Is a board foot the same as a square foot?
Only when the board is exactly 1 inch thick. Board feet are volume. Square feet are the face. An 8/4 board, 2 inches thick, has twice the board feet of a 1 inch board with the same face.