If you have ever bought hardwood or ordered rough lumber, you have seen prices quoted per board foot rather than per piece. It catches a lot of people off guard, because a board foot is a measure of volume, not length. Once you understand it, comparing lumber prices becomes easy.
The definition
A board foot is a unit of volume equal to a piece of wood 12 inches wide, 12 inches long, and 1 inch thick. That works out to 144 cubic inches. Any board with that same volume is one board foot, whether it is a wide thin plank or a narrow thick one.
Because it measures volume, a board foot tells you how much actual wood you are buying, which is why lumber yards use it to price boards of different widths and thicknesses fairly.
The formula
To calculate board feet, use the thickness and width in inches and the length in feet:
Board feet = (thickness in × width in × length ft) ÷ 12
You divide by 12 because there are 12 inches in the foot of length. If you prefer to work entirely in inches, use length in inches and divide by 144 instead.
Worked examples
A 2 inch by 8 inch board that is 10 feet long: 2 times 8 times 10, divided by 12, is about 13.3 board feet.
A 1 inch by 6 inch board that is 8 feet long: 1 times 6 times 8, divided by 12, is 4 board feet.
If you are buying several of the same board, calculate one and multiply by the quantity. Ten of those 1 by 6 boards would be 40 board feet.
Nominal size versus actual size
Here is the part that trips people up. A “2 by 4” is not actually 2 inches by 4 inches. That is the nominal size, the rough dimension before the board is dried and planed smooth. The actual size of a finished 2 by 4 is about 1.5 by 3.5 inches.
For construction lumber priced by the piece, this does not matter for your board foot math, because dimensional lumber is usually sold and priced per piece, not per board foot. But for hardwood and rough-sawn lumber, which is where board feet really come into play, you often measure the actual dimensions. Rough hardwood is frequently sold in quarter-inch thickness steps, written as 4/4 (one inch), 5/4, 6/4, and 8/4. A 4/4 board is a nominal one inch thick.
When in doubt, ask the yard whether they price on nominal or actual dimensions, since it changes the total.
Why it matters for a project
Board feet let you do two things quickly:
- Compare prices. A wide plank and a narrow one can be priced per board foot so you know which is truly cheaper per unit of wood.
- Estimate a whole order. Add up the board feet for every piece on your cut list, then multiply by the price per board foot for a total material cost before you leave home.
Let the calculator do the arithmetic
Our board foot calculator takes the thickness, width, length, and quantity and returns the total board feet, so you can price an order in seconds. For framing projects measured by the piece, the lumber calculator and the framing lumber calculator handle studs, plates, and joists directly.