What a board foot actually measures
A board foot is a unit of volume, not length or area. One board foot equals 144 cubic inches of wood — the same volume as a board 12 in wide, 12 in long, and 1 in thick (12 × 12 × 1). It's the standard unit for pricing and ordering solid lumber in the US, especially hardwood, because it accounts for thickness as well as width and length, unlike a simple linear-foot count.
That matters because two boards of the same length can hold very different amounts of wood. An 8-foot board that's 1 in thick and 6 in wide contains far less material than an 8-foot board that's 2 in thick and 12 in wide, even though both are "8 feet long." Board feet capture that difference; a linear-foot count does not.
The formula and a worked example
The calculation, when your length is in feet:
Board feet = (Thickness in × Width in × Length ft) ÷ 12
If you measured the length in inches instead, use 144 in the denominator rather than 12 — the two versions give the same answer, since dividing length by 12 first (to get feet) and then by 12 again is the same as dividing by 144 once.
Worked example: an 8-ft board of 4/4 (1 in thick) lumber, 6 in wide:
1 × 6 × 8 ÷ 12 = 4 board feet
Multiply the per-board result by how many boards you're buying to get the total. Ten of those boards would be 40 board feet, and at $7.00 per board foot that's a $280 order — exactly what this calculator works out for you automatically.
The quarters system: 4/4, 6/4, 8/4
Hardwood dealers describe thickness in quarters of an inch rather than plain inches — a habit that goes back to how rough lumber has long been sawn and billed. 4/4 ("four-quarter") is 1 in, 5/4 is 1.25 in, 6/4 is 1.5 in, 8/4 is 2 in, and so on up through 10/4 (2.5 in) and 12/4 (3 in) for thick table legs and turning stock.
The important part: board feet are figured on the rough, un-surfaced thickness — the size the board was sawn to at the mill — not the smoother, thinner size it ends up after planing. A board sold as 4/4 starts close to a full 1 in thick, but once it's surfaced on both faces (S2S) for retail sale, planing typically brings it down to around 13/16 in. It's still priced and labeled as 4/4 even though you'll actually measure something thinner in your hand. This is exactly why you should select the nominal quarter size in the calculator above, not the thickness you measure with a caliper after the fact.
Board feet vs. linear feet
Linear feet just measures length, with no regard for width or thickness — useful for things like trim or molding that come in one fixed profile. Board feet measures volume, so it changes with all three dimensions.
Two boards can have identical linear footage and very different board footage: an 8-ft board that's 6 in wide is 8 linear feet and 4 board feet, while an 8-ft board that's 12 in wide is also 8 linear feet but 8 board feet — twice the wood, twice the price, same length. Whenever width or thickness varies between boards, linear feet alone will mislead you on quantity or cost; board feet won't.
Estimating how much to buy
Once you know the board feet a project's finished parts require, don't order that exact amount. A widely used guideline is to add 15–30% on top, particularly for hardwood, to absorb:
- Defects and character in the rough stock — knots, checks, bark inclusions, and areas you'll want to cut around.
- Crosscutting and squaring waste, since rough lumber rarely comes perfectly straight or clean at the ends.
- Grain and color matching, when a project needs boards that look consistent next to each other.
Simple projects with long, straight parts can stay near 15%; projects built from many short pieces, or from figured or irregular hardwood, are safer at 25–30%. Enter your quantity above and multiply the resulting total board feet by your chosen buffer when you place the order.