A board foot is 144 cubic inches of lumber. To calculate board feet, multiply thickness in inches by width in inches by length in feet, then divide by 12. For several identical boards, multiply that result by the number of pieces. Before estimating a bill, check which dimensions the supplier uses for its tally.
What does one board foot actually mean?
Picture a piece of wood 12 inches wide, 12 inches long and 1 inch thick. That is one board foot, often abbreviated BF. The unit measures volume, so a narrower, longer board can contain the same amount of wood. A piece 6 inches wide, 24 inches long and 1 inch thick also contains one board foot.
The American Hardwood Export Council’s measurement guide explains this unit and the industry’s use of inches and feet. Board footage helps compare quantities, but it does not tell you whether individual boards have the dimensions, appearance or properties needed for a particular project.
How do you calculate board feet?
Board feet = thickness (inches) × width (inches) × length (feet) ÷ 12.
Suppose your calculation uses a thickness of 1 inch, a width of 8 inches and a length of 9 feet. Multiply 1 × 8 × 9 to get 72, then divide by 12. The result is 6 board feet. Five identical pieces total 5 × 6 = 30 board feet.
If every dimension is in inches, use thickness × width × length ÷ 144. The same board is 108 inches long, so 1 × 8 × 108 ÷ 144 still equals 6. Advantage Lumber’s calculator guide presents both versions. Choose one and label your units before entering numbers.
For a length of 7 feet 6 inches, enter 7.5 feet in the first formula or 90 inches in the second. Do not enter 7.6 feet: six inches is half a foot. For metric measurements, convert thickness and width to inches and length to feet first, or convert all three to inches. Our millimeters-to-inches guide explains that conversion.
Should you use actual or nominal dimensions?
For the physical volume of a rectangular board, use its measured dimensions. For a supplier’s price calculation, use the supplier’s stated billing dimensions and tally rules. These can give different answers without either calculation being an arithmetic mistake.
Hardwood thickness labels often use quarters of an inch: 4/4 represents 1 inch, 5/4 represents 1.25 inches and 8/4 represents 2 inches. AHEC distinguishes rough thickness from surfaced thickness. Its published convention subtracts 3/16 inch for surfaced lumber with nominal thickness of 1½ inches or less. Under that convention, 4/4 corresponds to a surfaced thickness of 13/16 inch.
Do not automatically replace the invoice’s thickness with that finished measurement. Nor should you assume a familiar size label always describes the dimensions you can measure. Ask the seller: “Which thickness, width and length go into your board-foot tally, and how do you round them?” AHEC also describes net and gross tallies, so ask which applies if those terms appear on the quote.
How do you total a mixed group of boards?
Give each distinct size its own row. Calculate the board feet for one piece, multiply by its quantity, then add the row totals. The following is an arithmetic example using the dimensions shown, not a supplier’s formal tally.
| Thickness × width × length | Pieces | BF per piece | Row total |
|---|---|---|---|
| 1 in × 8 in × 9 ft | 3 | 6 | 18 BF |
| 1.25 in × 6 in × 8 ft | 4 | 5 | 20 BF |
| 2 in × 9 in × 6 ft | 2 | 9 | 18 BF |
| Total | 9 | — | 56 BF |
A simple worksheet needs columns for thickness, width, length, quantity and board feet. Record the measurement basis beside the numbers, such as “measured dimensions” or “supplier’s quoted tally.” That note makes it easier to explain a difference later. For irregular boards, ask how the seller measures their width rather than treating the widest point as representative of the whole piece.
How do you estimate the lumber cost?
Estimated lumber subtotal = billed board feet × price per board foot. At an illustrative $7.50 per BF, a 56-BF tally produces a $420 subtotal. This is an invented example, not a current market price. Taxes, delivery, preparation charges and other fees may be separate.
If different rows have different prices, multiply each row by its own rate before adding the costs. Keep the quoted quantity separate from any planning allowance. For example, a chosen 10% allowance on 56 BF would add 5.6 BF, for 61.6 BF. That percentage is only a demonstration; volume alone cannot determine how much usable material a project needs.
Are board feet, linear feet and square feet interchangeable?
A linear foot measures length. A square foot measures area. A board foot measures volume. For a board measuring 1 inch thick, 6 inches wide and 8 feet long, the length is 8 linear feet, one broad face covers 4 square feet, and the volume is 4 board feet.
Double that board’s thickness to 2 inches and it still has 8 linear feet of length and a 4-square-foot broad face, but now contains 8 board feet. You therefore need thickness and width to convert linear feet into board feet. For more on separating dimensions, see our guide to volume calculations.
What should you check before trusting the total?
Check the length unit, divisor, quantity and measurement basis. An all-inches calculation divided by 12 instead of 144 is twelve times too large. Keep extra decimal places while adding rows, then round the displayed total. Finally, compare your worksheet with the seller’s itemized tally rather than expecting a geometric estimate to reproduce every commercial rounding rule.
For basic measuring supplies, you can browse inch rulers and tape measures on Amazon. Check the markings and measurement range against what you need.
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