A board foot measures lumber volume, not distance. One board foot is 144 cubic inches: the volume of a piece 1 inch thick, 12 inches wide, and 12 inches long. This common volume basis helps compare boards of different sizes, but the dimensions a supplier uses for pricing still matter.
What a Board Foot Measures
The reference volume can be written as 1 inch × 12 inches × 12 inches = 144 cubic inches. Other combinations of thickness, width, and length can contain the same volume.
Because board feet measure three-dimensional volume, a board's linear length by itself cannot determine its board feet.
Formula and Units
Use inches for thickness and width and feet for length in the common imperial formula. For multiple identical boards, calculate one board and multiply that unrounded value by the quantity.
Keep full precision while combining size groups, then round the final purchase estimate as needed.
Board feet = (thickness in inches × width in inches × length in feet) / 12
Total board feet for identical boards = single-board board feet × quantity
Choose the Dimensions That Match the Purchase
A nominal label, a rough-sawn size, a surfaced or dressed size, and a measurement taken from the stock can describe different stages or conventions. Suppliers may also base board-foot pricing on different stated dimensions, so do not assume one convention applies to every quote.
Confirm whether the seller is tallying rough dimensions, surfaced dimensions, nominal labels, or measured stock. For a part or cut-list estimate, measured actual thickness and width are usually the useful values.
For the terminology behind those differences, read Nominal vs Actual Lumber Sizes.
Board Feet and Linear Feet Answer Different Questions
Linear feet record length along a board. Board feet include thickness and width as well as length, so two boards with the same linear footage can contain different volumes.
See Board Feet vs Linear Feet for a focused comparison and conversion explanation.
To turn a board-foot quote into a length of a given size, or back, use the board foot to linear foot calculator.
A Purchasing Workflow for Mixed Stock
Separate the order into groups before doing arithmetic. This preserves the pricing basis for each size and makes a supplier comparison easier to audit.
- List each board-size group and its quantity.
- Identify the dimensions the supplier uses to tally or price that group.
- Calculate the board feet for each group without rounding each board early.
- Add the group totals.
- Add a documented purchasing allowance only when material condition, milling, selection, or layout makes one appropriate.
- Compare quotes only after confirming that each supplier uses the same pricing basis.
Worked Example: Two Board-Size Groups
This illustrative order contains three boards at 1.25 inches × 6 inches × 8 feet and two boards at 2 inches × 4 inches × 6 feet.
Group A: one board is (1.25 × 6 × 8) / 12 = 5 board feet. Three boards contain 5 × 3 = 15 board feet.
Group B: one board is (2 × 4 × 6) / 12 = 4 board feet. Two boards contain 4 × 2 = 8 board feet.
The combined volume is 15 + 8 = 23 board feet before any separately chosen purchasing allowance.
Metric Measurements
Metric dimensions can be converted to the equivalent board-foot volume, but the board foot remains an imperial volume convention. Convert all dimensions consistently instead of mixing millimetres with the inches-and-feet formula.
The Board Foot Calculator accepts mixed metric or imperial inputs and can add an optional cost estimate after the volume is calculated.
Common Mistakes
Most errors come from using a dimension or pricing basis without identifying what it represents. A written size-group tally makes the assumptions visible.
- Using nominal labels without checking the supplier's pricing dimensions.
- Confusing linear feet with board feet.
- Mixing inches and feet in the imperial formula.
- Including quantity in a group formula and then multiplying by quantity a second time.
- Rounding every board before adding groups.
- Assuming one waste or purchasing-allowance percentage suits every order.
Common questions
Can a differently shaped board still equal one board foot?
Yes. Any rectangular lumber volume equal to 144 cubic inches is one board foot; it does not have to be shaped as 1 inch × 12 inches × 12 inches.
How should I total boards with different dimensions?
Group identical sizes, calculate each group's unrounded board-foot volume, and add the group totals before rounding.
Is there one standard purchasing allowance?
No. Any allowance should reflect the particular stock, defects, milling, selection, and layout rather than a universal percentage.
Sources
- Valuing Standing Timber · Penn State Extension
Defines one board foot as 144 cubic inches and publishes the inches-and-feet formula used above.
- Northwest forest-industry data documentation · U.S. Forest Service
Also defines the unit as 12 inches × 12 inches × 1 inch.