What is a board foot?
A board foot is 144 cubic inches of lumber — the wood in a board 1 inch thick, 12 inches wide and 1 foot long. It is a measure ofvolume, which is the part that catches people out: it says nothing about how long a board is or what shape it comes in, only how much wood you are buying. Our board foot calculator does the arithmetic; this page explains what the number means and the two conventions that make it confusing.
The formula
Board feet = thickness (in) × width (in) × length (ft) ÷ 12.If the length is in inches instead of feet, divide by 144 — the cubic inches in a board foot — rather than 12. Worked through:
- The definition itself: 1 in × 12 in × 1 ft ÷ 12 =1 board foot
- A nominal 2×4, 8 ft long: 2 × 4 × 8 ÷ 12 =5.33 board feet
- Ten of those: 53.33 board feet
- A 4/4 board, 6 in wide, 8 ft long: 1 × 6 × 8 ÷ 12 =4 board feet
- The same board in 5/4: 1.25 × 6 × 8 ÷ 12 =5 board feet — 25% more wood, 25% more cost
Because it is a volume, a board foot of 1-inch stock covers twelve times the area of a board foot of 12-inch-thick timber. Lumber is priced per board foot, so the unit is really a way of charging for wood by the cubic inch without anyone having to say "cubic inch".
Trap one: hardwood quarters
Hardwood thickness is quoted in quarters of an inch and spoken as fractions — "four quarter", "eight quarter". Those are rough sawnthicknesses, measured as the board came off the saw. Surfacing planes material away, so the board you collect is thinner than the number on the invoice, and you are still charged for the rough dimension:
| Called | Rough (billed) | Typical surfaced |
|---|---|---|
| 4/4 | 1 in | 3/4 in |
| 5/4 | 1.25 in | 1 in |
| 6/4 | 1.5 in | 1 1/4 in |
| 8/4 | 2 in | 1 3/4 in |
| 12/4 | 3 in | 2 3/4 in |
Use the rough figure for board feet and money; use the measured thickness for anything that has to fit.
Trap two: nominal softwood
A surfaced 2×4 measures about 1.5 × 3.5 inches, a 1×6 about 0.75 × 5.5. Yet board feet for dimensional softwood are conventionally calculated on thenominal size — a 2×4 counts as 2 × 4. Both conventions exist for the same reason: the mill sawed the bigger dimension and planing is a service performed on wood you already bought. It does mean two people can compute different board feet for the same stack and both be defensible, which is why the calculator asks explicitly whether your figures are nominal or actual and applies the standard sizes rather than leaving you to guess.
Board feet, linear feet and square feet
Three units, constantly confused. Linear feet measure length only — fine when every board is the same section, useless for comparing prices across sizes. Square feet measure surface area, which is what flooring and sheet goods are sold by. Board feet measure volume, so they are the only one of the three that tells you how much actual timber you are getting. If a yard quotes per linear foot, convert to board feet before comparing it against a per-board-foot price, or you are comparing nothing at all.
Frequently asked questions
What is a board foot?
A board foot is a unit of lumber volume equal to 144 cubic inches — the amount of wood in a board 1 inch thick, 12 inches wide and 1 foot long. It measures volume, not area or length, which is why a board foot of thin wide stock and a board foot of thick narrow stock cost the same. The formula is thickness (in) × width (in) × length (ft) ÷ 12.
How do you calculate board feet?
Multiply thickness in inches by width in inches by length in feet, then divide by 12. A nominal 2×4 that is 8 feet long is 2 × 4 × 8 ÷ 12 = 5.33 board feet. If your length is in inches instead, divide by 144 rather than 12. For several identical pieces, work out one and multiply — ten of those 2×4s is 53.33 board feet.
What does 4/4 lumber mean?
It is hardwood thickness written in quarters of an inch, read aloud as "four quarter". 4/4 is one inch, 5/4 is an inch and a quarter, 8/4 is two inches. The catch is that those are rough sawn thicknesses: once the mill surfaces the board, 4/4 typically finishes at about 3/4 inch and 5/4 at about 1 inch — but you are charged for the rough dimension, because that is the wood the mill cut.
How thick is 5/4 lumber really?
Nominally 1.25 inches, and surfaced-two-sides it usually measures about 1 inch. Both numbers are correct for different purposes: use 1.25 in when calculating board feet and paying for it, and the actual measured thickness when designing joinery that has to fit. Confusing the two is the most common board-foot error there is.
Is a 2×4 really 2 inches by 4 inches?
No — a surfaced softwood 2×4 measures about 1.5 × 3.5 inches. But board feet for dimensional softwood are conventionally computed on the nominal size, so a 2×4 counts as 2 × 4. That convention is why our board foot calculator asks whether you are entering nominal or actual dimensions, and applies the standard nominal-to-actual sizes for you.
What does S4S mean?
Surfaced four sides — planed smooth on both faces and both edges, so it arrives ready to use and dimensionally consistent. S2S is surfaced two sides (faces planed, edges rough) and rough sawn is unplaned. Surfacing removes material, which is why S4S boards measure less than their nominal thickness while still being sold and priced by the nominal board foot.
How much does a board foot cost?
It depends entirely on species, grade and thickness — construction softwood is a fraction of the price of figured hardwood, and wider or thicker stock commands a premium because it comes from bigger logs. Rather than trust a national average, get the per-board-foot price from your local yard and multiply by your total; the calculator does that arithmetic alongside the volume.
About these numbers
The 144-cubic-inch definition and the nominal-dimension convention are industry standard, following the NHLA rules for hardwood measurement and the USDA Forest Products Laboratory's Wood Handbook; the surfaced sizes are the customary mill figures, which is why they are described as typical rather than exact. Every worked figure on this page is computed by the same tested module the board foot calculatoruses, so the two can never disagree — readhow CubicMath works, or see thematerial reference for the rest of our sourced constants.