What a board foot actually measures
A board foot is a unit of volume, not area or length. One board foot is 144 cubic inches — the amount of wood in a piece 1 inch thick, 12 inches wide, and 12 inches long. Picture a square foot of 1-inch lumber and you have it. That means a 1x12 that is 8 feet long contains eight board feet, while a 1x4 of the same length contains only about 2.7, even though both are the same number of linear feet.
Lumber yards use board feet because it lets them price wood of any dimension on one scale. A wide, thick slab and a stack of narrow strips can be compared directly once both are expressed in board feet. Softwood at a home center is usually tagged per piece, but hardwood dealers almost always quote a price per board foot, and rough stock is sold that way too.
The board foot formula
With every measurement in inches:
Here
For more than one identical piece, multiply the result by the quantity. That is exactly what the calculator above does — board feet per piece first, then the total, then the cost if you enter a price.
Worked example
The calculator’s default inputs are a board 1 inch thick, 6 inches wide, and 96 inches (8 feet) long, one piece, at $4.50 per board foot.
So one board is 4 board feet. With a quantity of one, the total is also 4.000 bf. The estimated cost is:
The same board, length in feet
Using the divide-by-12 version with the length as 8 feet:
Same answer, which is the point — the two forms of the formula are identical, just scaled for whichever length unit you have on hand.
Board feet by common lumber size
All rows below are 8-foot lengths, using nominal thickness and width.
| Nominal size | Board feet (8 ft) |
|---|---|
| 1x4 | 2.67 |
| 1x6 | 4.00 |
| 1x8 | 5.33 |
| 1x10 | 6.67 |
| 1x12 | 8.00 |
| 2x4 | 5.33 |
| 2x6 | 8.00 |
| 2x8 | 10.67 |
| 2x10 | 13.33 |
| 2x12 | 16.00 |
Halve these numbers for a 4-foot piece, or scale proportionally for any other length — a 1x6 at 12 feet is 6.0 board feet, at 10 feet it is 5.0.
Nominal vs. actual dimensions
This is where board foot math trips people up. A construction "2x4" is not actually 2 inches by 4; after drying and planing it measures about 1.5 by 3.5. But it is priced and sold as a 2x4, so the board foot count that matches the price uses the nominal 2 and 4. Use nominal size for any surfaced softwood you buy dimensioned.
Rough-sawn hardwood is different. It is graded and sold at its actual measured size, and hardwood thickness is quoted in quarters of an inch:
- 4/4 — 1 inch
- 5/4 — 1.25 inches
- 6/4 — 1.5 inches
- 8/4 — 2 inches
A 4/4 board 6 inches wide and 8 feet long is 4 board feet. The same footprint in 8/4 stock is 8 board feet, because it is twice as thick. When a dealer measures a random-width board, they typically round the width to the nearest inch (or half inch) before figuring the board feet.
Estimating cost and how much to buy
Cost is simply board feet times the price per board foot, which the calculator handles when you fill in the price field. The number it returns is the net material, though — the wood that ends up in the project. Real buying always needs a margin on top:
- Add 15–20% for a straightforward hardwood project with mostly long, clear parts.
- Add 30–40% when parts are short, when you are working around knots and checks in lower grades, or when grain has to match across pieces.
- Round up to whole boards. You cannot buy 4.6 board feet of a single plank; you buy the plank.
For an idea of how rough lumber loses dimension as it dries and is milled — which affects how much you need to start with — see the lumber shrinkage calculator, and the guide on board feet explained walks through the concept with more examples.
Common mistakes
- Using actual size for softwood. Plugging 1.5 x 3.5 into the formula for a 2x4 undercounts the board feet you are paying for by roughly a third.
- Forgetting to convert feet to inches. Entering 8 for the length instead of 96 gives an answer twelve times too small. Use the divide-by-12 formula if you want to keep feet.
- Confusing board feet with linear feet. A quote of "$3 per foot" and "$3 per board foot" are very different numbers for anything wider than a 1x12.
- Ignoring waste. Ordering the exact board-foot total leaves nothing for saw kerf, defects, or a mismeasured cut.
- Mixing thickness units. If you are buying 5/4 stock, the thickness in the formula is 1.25, not 1.