How this instrument works
A board foot is a volume measurement, not a length or an area measurement, even though the name sounds like it should be linear. One board foot equals the volume of a piece 1 inch thick, 12 inches wide, and 12 inches long — 144 cubic inches, or exactly 1/12 of a cubic foot. Any board's board-foot volume comes from multiplying its thickness in inches by its width in inches by its length in feet, then dividing by 12.
Lumber yards and sawmills price rough and dimensional lumber by the board foot because it captures the actual wood volume in a piece of stock regardless of its individual dimensions — a short, thick board and a long, thin piece can contain the same board footage and cost roughly the same, even though their lengths and widths look nothing alike side by side.
The formula works from nominal or actual dimensions depending on what's being priced: rough-sawn lumber sold by the board foot typically uses nominal sizes (a '2x6' entered as 2 in x 6 in), even though the actual dry, planed dimensions of a finished 2x6 are closer to 1.5 in x 5.5 in. Check which convention a supplier is quoting before entering dimensions, since the two can differ by 20 percent or more on stock labeled identically.
Board footage scales linearly with each of the three dimensions, so doubling any one of thickness, width, or length doubles the total board feet, while doubling two of them quadruples it. A thick 4x4 post can rack up more board feet per running foot than a wide, thin 1x8, even though the 1x8 looks like it uses more material at a glance.
- Enter the board's dimensional thickness into Thickness (in) — 2 for a 2x6, for example.
- Enter the board's dimensional width into Width (in) — 6 for a 2x6.
- Enter the board's length into Length (ft) — lumber length is conventionally measured in feet, not inches.
- Read Board feet beneath the inputs — the volume figure lumber is typically priced by, per piece or per thousand board feet.
Worked example — a 2x6, 8 ft long
Enter 2 into Thickness (in), 6 into Width (in), and 8 into Length (ft) for a nominal 2x6 board 8 feet long. The instrument multiplies 2 x 6 x 8 = 96, then divides by 12: 96 / 12 = 8.0. Board feet reads 8.0 bf.
That single 2x6x8 piece carries the same 8.0 board feet as, say, a 1x4 that's 24 ft long (1 x 4 x 24 = 96, / 12 = 8.0) — two boards that look nothing alike side by side but contain the identical wood volume, which is exactly why lumber gets priced by board footage rather than by length alone.
Questions
Is a board foot a measure of length, area, or volume?
Volume. Despite the name, a board foot is not a linear foot of board — it equals 144 cubic inches, the volume of a piece 1 inch thick, 12 inches wide, and 12 inches long, or exactly 1/12 of a cubic foot. Multiplying a board's thickness, width, and length together and dividing by 12 converts those three dimensions into that single volume figure.
Should I use nominal or actual lumber dimensions?
It depends on how your supplier prices the stock — check before entering numbers. Rough-sawn lumber sold by the board foot is typically quoted using nominal sizes (a '2x6' entered as 2 in x 6 in), while a finished, dry, planed 2x6 actually measures closer to 1.5 in x 5.5 in. The two conventions can produce board-footage figures that differ by 20 percent or more for boards labeled identically.
Why is lumber priced by board foot instead of by length?
Because board footage captures the actual volume of wood in a piece, regardless of its individual shape — a short, thick post and a long, thin plank can contain the same board footage and represent roughly the same amount of raw material, even though a simple linear-foot price would treat them very differently. Pricing by volume keeps the cost tied to the wood actually used.
How many board feet are in a standard 2x4?
It depends entirely on length, since lumber volume scales directly with it. An 8 ft long nominal 2x4 works out to (2 x 4 x 8)/12 = 5.33 board feet; a 10 ft length of the same 2x4 comes to (2 x 4 x 10)/12 = 6.67 board feet. Enter the specific length on hand rather than assuming a fixed board-foot figure for '2x4' in general.
Does doubling a board's width double its board footage?
Yes — board footage scales linearly with each individual dimension, so doubling width alone doubles the board feet, doubling length alone doubles it, and doubling any two of the three dimensions at once quadruples it. All three dimensions carry equal mathematical weight in the formula, even though width and thickness are usually the smaller numbers.
Does this calculation account for kerf loss or milling waste?
No — it returns the theoretical board footage of the stock dimensions entered, not the usable yield after saw kerf, planing, or defect removal. Real milling and cutting operations typically lose some material to the saw blade's width and to squaring up rough stock, so purchased board footage should include an allowance beyond the net board footage a project actually needs.