What a board foot actually measures
A board foot is a unit of volume, not of area or length. One board foot is 144 cubic inches — the amount of wood in a piece 1 inch thick, 12 inches wide and 1 foot long. That is exactly one-twelfth of a cubic foot, and 0.0023597 cubic metres.
Because it is a volume, the same board foot can arrive in very different shapes. A 1 in × 12 in × 1 ft board, a 2 in × 6 in × 1 ft block and a 1 in × 6 in × 2 ft strip are all one board foot. That is the whole point of the unit: it lets a mill price 4/4 cherry, 8/4 maple and 12/4 white oak on one consistent scale, even though the boards are nothing alike.
Softwood dimension lumber for framing is usually sold by the piece — you ask for forty 2×6×12s, not for 480 board feet. Hardwood, rough-sawn stock, timbers and anything bought from a sawmill is nearly always sold by the board foot, and wholesale loads are quoted per thousand board feet, written MBF (the M is the Roman numeral for a thousand, not “million”). Getting that distinction wrong is a factor-of-1,000 error on a purchase order.
The formula, and why you divide by twelve
Multiply thickness in inches by width in inches by length in feet, then divide by 12. The division by twelve is a unit conversion and nothing more: thickness × width gives square inches, and there are 144 square inches in a square foot, so if the length were also in inches you would divide by 144. Because you supply the length in feet, one factor of 12 has already been applied and only the other one is left.
Both forms are on this page. Use T × W × L ÷ 144 when every dimension is in inches — handy for short offcuts and for turning blanks. Use T × W × L ÷ 12 when the length is in feet, which is how lumber is normally described.
The variable that catches people is thickness. Hardwood thickness is quoted in quarters of an inch of rough thickness: 4/4 is one inch, 5/4 is 1¼, 6/4 is 1½, 8/4 is two inches, and 12/4 is three. You are charged on that rough figure even after the mill surfaces the board down to 13/16 in or 1¾ in, because the wood removed by the planer was wood you bought. This calculator accepts thickness directly in quarters for that reason.
Softwood works the same way through a different mechanism. Under the American Softwood Lumber Standard, PS 20, a nominal 2×4 is dressed to an actual 1½ in × 3½ in, yet every board-foot count in the trade uses the nominal 2 × 4. If you enter 1.5 and 3.5 you will get the net volume of the stick, which is about 66% of the board-foot figure your supplier will invoice.
Worked example: twelve pieces of 8/4 walnut
You are buying twelve rough-sawn black walnut boards, 8/4 thick, 7 in wide and 9 ft long, at $12.50 per board foot, and you want 15% extra for defects and cut-off.
- Convert the thickness. 8/4 = 8 ÷ 4 = 2 in.
- Cross-section. 2 in × 7 in = 14 in².
- Board feet per piece. 14 × 9 ft ÷ 12 = 10.5 BF.
- Total board feet. 10.5 × 12 pieces = 126 BF.
- Add the allowance. 126 × 1.15 = 144.9 BF to order.
- Cost. 144.9 × $12.50 = $1,811.25.
- Cross-check in MBF. 144.9 ÷ 1,000 = 0.1449 MBF. At an equivalent $12,500 per MBF that is the same $1,811.25.
- Other units. The order is 12 × 9 = 108 lineal feet, and 144.9 × 0.0023597 = 0.342 m³.
Now notice what the surfacing costs you. If that walnut comes back from the planer at 1¾ in by 6¾ in, the net volume is 1.75 × 6.75 × 9 ÷ 12 = 8.86 BF per piece against the 10.5 BF you paid for. You bought 126 BF and you will hold 106 BF of finished stock. That is normal, expected, and priced into the rough figure — but it is why a project cut list in finished dimensions always has to be grossed up before you go shopping.
How to read the result when you are comparing prices
Compare suppliers on dollars per board foot of the same thickness and grade, never on the price of a board. A wide board and a narrow board of the same species can differ by 40% per board foot because width itself is graded and priced; that is a real difference, not a mistake.
Three checks are worth making before you accept a quote. First, confirm the basis: $/BF and $/MBF differ by exactly 1,000, so $650/MBF is $0.65/BF. Second, confirm whether the price is for rough or surfaced stock, and if surfaced, whether the tally was taken before or after planing — the honest convention is to tally rough. Third, ask how random widths are measured: under National Hardwood Lumber Association rules the surface measure of a board is its width in inches times its length in feet divided by 12, with the width taken to the nearest inch, and that rounding can move a tally by a percent or two on narrow stock.
For a yield sanity check, a rule of thumb that carpenters use is that you need roughly 20% to 30% more rough board feet than the finished board feet in your cut list once you allow for planing, jointing, defect cut-out and end trim. Treat that as a starting allowance and refine it from your own experience with the species and grade you buy; the construction waste factor calculator lets you apply a measured factor across a whole takeoff.
Board feet in common nominal sizes
| Nominal size | BF per lineal ft | 8 ft | 10 ft | 12 ft | 16 ft |
|---|---|---|---|---|---|
| 1×4 | 0.333 | 2.67 | 3.33 | 4.00 | 5.33 |
| 1×6 | 0.500 | 4.00 | 5.00 | 6.00 | 8.00 |
| 1×12 | 1.000 | 8.00 | 10.00 | 12.00 | 16.00 |
| 2×4 | 0.667 | 5.33 | 6.67 | 8.00 | 10.67 |
| 2×6 | 1.000 | 8.00 | 10.00 | 12.00 | 16.00 |
| 2×8 | 1.333 | 10.67 | 13.33 | 16.00 | 21.33 |
| 2×10 | 1.667 | 13.33 | 16.67 | 20.00 | 26.67 |
| 2×12 | 2.000 | 16.00 | 20.00 | 24.00 | 32.00 |
| 4×4 | 1.333 | 10.67 | 13.33 | 16.00 | 21.33 |
| 6×6 | 3.000 | 24.00 | 30.00 | 36.00 | 48.00 |
A 2×6 and a 1×12 both contain exactly one board foot per lineal foot, which makes them useful mental yardsticks.
Which rulebook applies to your tally
Hardwood is measured and graded under the National Hardwood Lumber Association Rules for the Measurement and Inspection of Hardwood and Cypress, which define surface measure, the quarter system for thickness, and how the clear-face cuttings in a board set its grade. Softwood dimension sizes come from the American Softwood Lumber Standard, PS 20, published through the U.S. Department of Commerce, which fixes the dressed sizes that go with each nominal size. Neither document changes the arithmetic on this page; they tell you which numbers to put into it.
Mistakes that throw a board-foot count off
- Using surfaced dimensions. Entering 1.5 × 3.5 instead of 2 × 4 gives the net volume of the stick, which is 66% of the board-foot figure you will be billed. This calculator flags the common dressed sizes when it sees them.
- Confusing MBF with millions. M is a thousand. A quote of $650/MBF is 65 cents a board foot.
- Mixing feet and inches in the same formula. Divide by 12 when the length is in feet and by 144 when it is in inches. Nothing else changes.
- Tallying a random-width lift as if every board were the widest one. Measure each board, or use the mill's tally sheet.
- Forgetting that you buy rough and build finished. Plan the cut list in finished sizes, then gross up for planing, jointing and defect cut-out before ordering.
- Applying a waste percentage twice. If your cut list already carries an allowance, do not add another one here.
- Assuming board feet tell you what a load weighs. They do not: a board foot of balsa and a board foot of white oak have the same volume and very different masses.
Board feet, lineal feet, square feet and cubic metres
Four measures compete for the same lumber, and each answers a different question.
Lineal (running) feet count length only and ignore the section. They are what you want for trim, decking and fencing where you are covering a distance: twelve 9 ft boards are 108 lineal feet regardless of thickness. Deck boards are counted this way in the deck board calculator, and pickets in the fence picket calculator.
Square feet count coverage and are the right unit for sheet goods and for flooring; the plywood sheet calculator works in sheets and square feet rather than board feet. Note that hardwood flooring is sold by the square foot of coverage after milling, so a board-foot conversion is not meaningful for it.
Cubic metres are the international timber trade unit. One cubic metre is 423.776 board feet, so a board foot is 0.0023597 m³. Mills exporting to Europe and Japan quote per cubic metre, and shipping cube is always metric.
For framing takeoffs you generally want piece counts rather than volume: the wall stud count calculator and the floor joist span and count calculator both return piece counts and lineal feet, which you can then convert here with the BF-per-lineal-foot column above.
Key terms
- Board foot (BF)
- 144 cubic inches of lumber; one-twelfth of a cubic foot. The trade unit for hardwood and rough stock.
- MBF
- One thousand board feet. The wholesale quoting unit. $/MBF ÷ 1,000 = $/BF.
- Quarters (4/4, 8/4)
- Hardwood rough thickness in quarter-inches. 4/4 is one inch, 8/4 is two inches. You are billed on the rough thickness.
- Nominal vs actual
- Nominal is the name of the size (2×4); actual is what it measures after drying and dressing (1½ × 3½). Board feet use nominal.
- Surface measure
- Under NHLA rules, a board's width in inches times its length in feet, divided by 12 — the board-foot count of a nominal 1 in board.
- Lineal foot
- One foot of length, whatever the section. Used for trim, decking and fencing.