Board feet = thickness (inches) × width (inches) × length (feet) ÷ 12. A 2x6 that runs 10 feet is 2 × 6 × 10 ÷ 12, which comes to 10 board feet.
Use the nominal size, the one on the tag, not what a tape measure shows. Getting that wrong undercounts an order by about a third.
What you are measuring
A board foot is a volume, not a length. One board foot is a piece 12 inches long, 12 inches wide, and 1 inch thick, which is 144 cubic inches of wood. Everything about the formula follows from that: the 12 you divide by is there to turn inches-times-inches-times- feet into board feet.
The unit exists so that lumber of different sizes can be priced off one number. A yard selling 1x6, 2x4, and 4x4 at three different prices per piece tells you nothing about which is the better deal. Put them all in board feet and it is one rate against another.
The formula, three ways
Which version you use depends on what units your length is in. They all give the same answer.
Length in feet: thickness (in) × width (in) × length (ft) ÷ 12
Length in inches: thickness (in) × width (in) × length (in) ÷ 144
Everything in feet: thickness (ft) × width (ft) × length (ft) × 12
The first one is the version you will use almost every time, because lumber is tagged with thickness and width in inches and length in feet.
Worked examples
A 2x4, 8 feet long: 2 × 4 × 8 = 64, divided by 12 is 5.33 board feet.
A 1x12, 10 feet long: 1 × 12 × 10 = 120, divided by 12 is 10 board feet. Note that a 1x12 gives you the same volume as a 2x6 of the same length, since 1 × 12 and 2 × 6 both come to 12.
A 4x4 post, 96 inches: the length is in inches, so 4 × 4 × 96 = 1,536, divided by 144 is 10.67 board feet. Convert 96 inches to 8 feet and run the first formula instead and you get the same 10.67.
Twenty 2x6x12s: one piece is 2 × 6 × 12 ÷ 12 = 12 board feet. Twenty of them is 240.
The mistake that costs a third of the order
A 2x4 does not measure 2 inches by 4 inches. It measures about 1.5 by 3.5, because the name refers to the rough-sawn size before the board was planed and dried. Every dimensional size works the same way: a 1x6 is really 0.75 by 5.5, a 2x10 is 1.5 by 9.25.
Board feet are figured on the nominal size, the name on the tag. So is the price. If you measure the actual board and run the formula, a 2x4x8 comes out at 3.5 board feet instead of 5.33, and your whole order lands about a third short of what the yard will charge you for. Use the tag.
Rough-sawn and hardwood
Hardwood is where you will meet board feet most often, and it is sold in a different vocabulary. Thickness is quoted in quarters of an inch: 4/4 is one inch, 5/4 is an inch and a quarter, 8/4 is two inches. Widths are often random instead of standard, so the yard measures each board and tallies the board feet for you.
The formula is the same. A 4/4 board that is 7 inches wide and 9 feet long is 1 × 7 × 9 ÷ 12, which is 5.25 board feet. With rough-sawn stock the tagged thickness is close to the real thickness, so the nominal-versus-actual problem mostly goes away.
The Lumber Calculator does the arithmetic for any size and quantity. For the common dimensional sizes already worked out, see how many board feet in a 2x4. And if the reason you are counting is to price the order, the Lumber Price Index shows which way softwood has moved this year.
Skip the arithmetic
Enter your own dimensions and get the exact figure instead of working from averages.
Open the Lumber CalculatorFrequently asked questions
Thickness in inches, times width in inches, times length in feet, divided by 12. A 2x6 that is 10 feet long is 2 x 6 x 10 / 12, which is 10 board feet. If your length is in inches too, divide by 144 instead of 12.
A unit of volume: a piece of wood one foot long, one foot wide, and one inch thick, which is 144 cubic inches. It is how lumber yards price hardwood and larger orders, because it lets a 1x8 and a 2x4 be compared on the same basis when a per-piece price cannot.
Nominal, the size printed on the tag. A 2x4 measures 1.5 by 3.5 inches once dressed, but board feet and pricing run off the 2x4 name. Measure the actual board and your count comes out about a third low, which is the most common way people undercount an order.
Work out one piece, multiply by how many you need, and repeat for each size. Add the totals. A yard will do the same thing when it prices you, so if your figure and theirs disagree, one of you has used actual dimensions or slipped a decimal on the length.
More guides
How Many Board Feet in a 2x4?
5.33 for an 8-footer, and a table for every common length and size you will find at the yard.
How Much Does a Gallon of Paint Cover?
One gallon covers 350 to 400 sq ft in a single coat, though your walls will probably do worse.
How Much Paint Do I Need?
Worked gallon counts for five common room sizes, plus the formula to run your own.
How Much Does 5 Gallons of Paint Cover?
A bucket covers about 1,750 sq ft in one coat. Whether that saves you money depends on the job.
How Many Shingles Are in a Bundle?
26 in a 3-tab bundle, about 21 in an architectural one. Neither is the number you order from.
How Many Bundles of Shingles Are in a Square?
Three bundles make a square on almost every asphalt shingle, but some designer lines take four.