The formula, and the one substitution that trips everyone
A board foot is 144 cubic inches of lumber: a piece one inch thick, twelve inches wide and one foot long. The working formula is thickness in inches times width in inches times length in feet, divided by twelve. If you have the length in inches instead, divide by 144 rather than 12.
The substitution people get wrong is which thickness and width go into that formula. For dimensional softwood — anything you buy as a 2x4, a 2x10, a 4x4 — you use the nominal numbers. A 2x10 twelve feet long is 2 times 10 times 12 divided by 12, which is 20 board feet. Nobody measures it and gets 20. The stick is 1-1/2 by 9-1/4, so its actual volume is 1.5 times 9.25 times 12 divided by 12, or 13.875 board feet. The tally is 31 percent larger than the wood. That is not a scam, it is a convention that survived from when lumber was tallied green and rough at the mill before drying and planing took it down. Every supplier uses it, so prices remain comparable.
For rough-sawn hardwood the convention flips. You measure the board and use what you measure, with thickness expressed in quarters of an inch: 4/4 is one inch, 8/4 is two inches. If the board has already been surfaced, you still pay for the rough thickness — a 4/4 board that arrives at 13/16 is billed as one inch thick, because the yard bought and dried it at one inch.
Nominal against actual, in one table
| Nominal | Actual | Bd ft per lineal foot | Real wood per lineal foot |
|---|---|---|---|
| 1x4 | 3/4 x 3-1/2 | 0.333 | 0.219 |
| 1x6 | 3/4 x 5-1/2 | 0.500 | 0.344 |
| 5/4x6 decking | 1 x 5-1/2 | 0.625 | 0.458 |
| 2x4 | 1-1/2 x 3-1/2 | 0.667 | 0.438 |
| 2x6 | 1-1/2 x 5-1/2 | 1.000 | 0.688 |
| 2x8 | 1-1/2 x 7-1/4 | 1.333 | 0.906 |
| 2x10 | 1-1/2 x 9-1/4 | 1.667 | 1.156 |
| 2x12 | 1-1/2 x 11-1/4 | 2.000 | 1.406 |
| 4x4 | 3-1/2 x 3-1/2 | 1.333 | 1.021 |
| 6x6 | 5-1/2 x 5-1/2 | 3.000 | 2.521 |
The deduction pattern is worth memorising: under six inches nominal, subtract half an inch; six inches and over, subtract three quarters. That is why a 2x6 is 5-1/2 but a 2x8 is 7-1/4 rather than 7-1/2. The pattern breaks for timbers over about six inches, where the deduction is half an inch again on rough-sawn stock from a small mill.
Where the yard's number will differ from this one
Hardwood tallies are not pure arithmetic. Most yards measure rough width to the nearest inch, so a 7-3/8 inch board tallies as 7. Some round the board-foot figure per piece, some round the invoice total. Random-width, random-length lots are usually sold by the tallied bundle rather than piece by piece, and the surcharges for wide boards, long boards and select grades sit on top. A calculated figure inside a few percent of the invoice is doing its job.
None of this covers what you actually get out of the wood. Yield after ripping past knots, sapwood and checks runs anywhere from 60 to 85 percent depending on grade and how forgiving your cut list is. Buy the board feet, then buy some more.
Questions people ask
How many board feet in a 2x4?
Two thirds of a board foot per lineal foot, so an eight-foot 2x4 is 5.33 board feet and a ten-footer is 6.67. The arithmetic is 2 times 4 times the length in feet, divided by 12. Note that this uses the nominal 2 and 4, not the actual 1-1/2 by 3-1/2 you would measure. The real volume of an eight-foot 2x4 is 1.5 times 3.5 times 8 divided by 12, or 3.5 board feet — about 34 percent less wood than the tally says. Both numbers are used in the trade, for different purposes: the nominal tally for buying and pricing, the actual dimensions for layout and fit.
What does 4/4 mean and how is it different from 1x?
Four quarters, meaning one inch of rough thickness, and it is a hardwood term. The difference from 1x is that 4/4 is what the board actually measured when it came off the saw, while 1x is a name for a piece that measures 3/4 after drying and planing. Buy 4/4 rough and you get a full inch to work with. Buy 4/4 surfaced two sides and you get about 13/16, but you are still billed for the inch. This matters when your project needs a finished 3/4 thickness with a face to clean up: 4/4 gives you room, 1x pine does not.
Why am I paying for wood that is not there?
Because the price per board foot already accounts for it. When a 2x10 is quoted at some figure per board foot, that figure was set in a market where everyone knows a 2x10 tallies 1.667 board feet per foot and contains 1.156. If the convention changed to actual dimensions tomorrow, the price per board foot would rise by about 44 percent and the price of the stick would not move. The nominal system only causes trouble when you mix it up — using nominal dimensions for a layout, or comparing a softwood price per board foot against a hardwood price per board foot as though they measured the same thing.
How do I convert board feet to lineal feet?
Divide the board feet by the board feet per lineal foot of that size, which is nominal thickness times nominal width divided by 12. For 2x6 that divisor is 1.0, so 100 board feet of 2x6 is 100 lineal feet. For 2x4 the divisor is 0.667, so 100 board feet is 150 lineal feet. For 2x10 it is 1.667, so 100 board feet is 60 lineal feet. Then convert lineal feet to pieces by dividing by the stock length you can buy, and round up per piece rather than in total, because you cannot buy two thirds of a stud.