A board foot is a volume, not a length
A board foot is the standard unit timber is bought and sold in across North America, and it trips people up because it sounds like a measure of length. It is not. One board foot is a volume — a piece one foot long, one foot wide and one inch thick, or 144 cubic inches of timber however that volume is arranged.
That definition is what makes the unit useful. A merchant selling both 2x4s and 1x8s needs one number that lets him price them against each other, and the length in feet does not do that. Volume does.
The formula follows straight from the definition, with the 12 in the denominator converting inches of thickness into feet.
Board feet = nominal width (in) × nominal thickness (in) × total length (ft) ÷ 12
So a single 8 foot 2x4 is 2 × 4 × 8 ÷ 12 = 5.33 board feet. Ten of them are 53.3 board feet.
The part that catches everyone out
Nominal sizes are not real sizes. A 2x4 does not measure two inches by four inches. It is sawn at roughly that size green, then dried and planed on all four faces, and what arrives on site measures 1½ by 3½ inches.
The size losses are consistent enough to be worth memorising.
- One inch nominal: finishes at ¾ inch. A 1x2 is really ¾ by 1½.
- Two, four and six inch nominal: lose half an inch. A 2x4 is 1½ by 3½, a 4x4 is 3½ by 3½, a 2x6 is 1½ by 5½.
- Eight inch nominal and above: lose ¾ of an inch. A 2x8 is 1½ by 7¼.
Here is the thing that surprises people. Board feet are still calculated on the **nominal** size, and that is correct, not a mistake anyone is making. The unit exists to price timber, and timber is priced on the size it was sawn at, because that is the size the mill actually cut out of the log. The planing waste is the mill's, but you pay for it either way — it is built into the market price per board foot.
So when you are ordering and costing, use nominal. Always.
Where the real size does matter
The dressed size matters the moment you stop pricing timber and start doing something physical with it.
- Weight and transport: a 2x4 has a real section of 1½ × 3½ = 5.25 square inches, against a nominal 8 square inches. That is about a third less material than the name implies, and a weight worked out from the nominal figure comes out roughly half as heavy again as the load really is. That is the difference between one trip and two, or between within the van's payload and over it.
- Setting out: three 2x4s laid flat give you 4½ inches of thickness, not 6. Openings, packers and stud walls all have to be set out from the real dimension.
- Structural design: span tables and load calculations use the dressed section, because that is the timber that is actually resisting the load.
The lumber calculator on this site follows exactly that split. Board feet come from the nominal size because that is what the merchant will bill, and the weight comes from the dressed size because that is what the vehicle has to carry.
A worked order
Say you are framing a 12 foot wall at 16 inch centres with 8 foot 2x4 studs in pine, and you want a 10 percent waste allowance.
The span and spacing give 10 studs — both ends carry one, so it is one more than the division suggests. Ten studs plus 10 percent, rounded up to whole boards, is 11 pieces. Eleven 8 foot pieces is 88 linear feet.
Board feet are 2 × 4 × 88 ÷ 12 = 58.7 board feet. That is the number to price.
The weight is a different calculation entirely. The real section of 1½ by 3½ inches over 88 feet works out at about 0.09 cubic metres, and pine at roughly 500 kilograms per cubic metre puts the load at about 45 kilograms, or 100 pounds. Cedar at 370 would be lighter, oak at 750 would be half as heavy again.
Practical points when ordering
Timber is stocked in fixed lengths, usually in two foot increments, so ask for the lengths that cut cleanly into what you need rather than buying long and wasting the offcuts. Where lengths are priced per piece rather than per board foot, do the board foot arithmetic anyway — it is the only way to compare two quotes for different sections honestly. And remember that a waste factor on framing is applied to the piece count and rounded up, because half a stud is no use to anyone.