Board Foot Calculator for Hardwood and Nominal Lumber

How to use this tool

Pick the size from the dropdown rather than typing numbers, because the dropdown is what makes the two columns of results mean anything. Hardwood in the quarter system sets the thickness and leaves you to type the width, since hardwood comes in random widths and is measured board by board. Softwood nominal sizes set both, because a 2×6 is a 2×6 wherever you buy it. Custom takes whatever you type and treats it as real measured timber.

The number you order against is the billed one, in the big card. The second card is the wood that will actually be in your hands, which is the number to use when you are working out whether the timber fills the space. Waste is applied to the billed figure, since that is what you are paying for. Fifteen percent is a sensible default for furniture work with defects to cut around, and you can drop it to five for clean construction lumber.

What a board foot is

A board foot is a volume: one foot long, one foot wide, one inch thick. That is 144 cubic inches. It is not a length and it is not an area, which is why comparing it to linear feet gets people into trouble.

The working formula is thickness in inches times width in inches times length in feet, all divided by 12. The 12 is there because the length is in feet while the other two are in inches, so you are converting one dimension. If you have all three in inches the divisor is 144 instead. Both routes give the same answer and the tool uses the first, printing the arithmetic so you can check it.

The consequence is the one drawn in the diagram. A 1 inch by 12 inch board 12 feet long is 12 board feet. So is a 2 inch by 6 inch board 12 feet long, because the cross section is the same 12 square inches. So is a 4 by 4 at 9 feet, because its larger 16 square inch section is carried over a shorter length. Three boards you could not confuse by eye, one number.

Why you are billed for wood you never receive

This is the part that makes people think they have been shortchanged, and it is worth understanding rather than arguing about at the counter.

Lumber is measured when it is rough sawn, before it is dried and planed. A board cut at a full two inches is a 2 inch board for pricing purposes forever after, even though drying shrinks it and surfacing takes another eighth or so off each face. By the time a nominal 2×4 reaches the rack it measures 1.5 by 3.5. You are billed on 2 by 4, which is 8 square inches of cross section, and you receive 5.25 square inches. That is 34 percent of the volume gone, and it is entirely legal and entirely standard.

Hardwood does the same thing more gently. A 4/4 board is billed as a full inch and typically arrives at 13/16 once it has been surfaced on two sides, so about 19 percent goes. Buy it rough and you keep the lot, which is why serious furniture makers buy rough and thickness it themselves. The width is measured board by board on hardwood rather than being nominal, so the loss there is thickness only.

None of this is a reason to be suspicious. It is a reason to know which figure you are looking at. Price comparisons between suppliers are meaningful because everyone quotes on the same billed basis. Comparisons between a billed board foot and the wood in your project are not, and the second card on this page exists so you never have to make that mistake.

A worked check you can repeat on paper

The tool loads with three lines. First, six pieces of 4/4 walnut, 6 inches wide and 8 feet long. Billed: 1 times 6 times 8 divided by 12 is 4 board feet each, so 24 for the six. Actual, at a surfaced 13/16: 0.8125 times 6 times 8 divided by 12 is 3.25 each, so 19.5. You paid for 24 and you have 19.5.

Second, ten 2×4 studs at 8 feet. Billed on the nominal 2 by 4: 2 times 4 times 8 divided by 12 is 5.3333 each, so 53.3333 for ten. Actual on 1.5 by 3.5: 1.5 times 3.5 times 8 divided by 12 is 3.5 each, so 35. That is the 34 percent gap, and on a framing order of any size it is a large number.

Third, four pieces of rough sawn oak measured at 1.125 by 7.25 by 10 feet. Rough means what you measure is what you get, so billed and actual are both 1.125 times 7.25 times 10 divided by 12, which is 6.7969 each and 27.1875 for four.

Add them: 24 plus 53.3333 plus 27.1875 is 104.5208 board feet billed, against 81.6875 actually delivered, a loss of 21.85 percent across the order. At the 15 percent waste allowance the tool starts with, you would buy 120.20 board feet.

Mistakes that catch people out

• Dividing by 12 when the length is in inches. The 12 assumes feet. With everything in inches use 144. Getting this wrong is a factor of twelve, which is obvious enough to catch, unlike most errors.
• Using the actual size of dressed lumber to price it. A 2×4 is billed as 2 by 4 even though it measures 1.5 by 3.5. Price it on the actual size and your quote comes in a third light.
• Using the billed size to work out whether it fits. The exact opposite error. Two 2x4s laid flat give 3 inches of thickness, not 4.
• Buying board feet of a fixed width. Hardwood comes in random widths. A 40 board foot order might be nine wide boards or fourteen narrow ones, and if your design needs a particular width you have to specify it and pay for it.
• Forgetting waste on figured or defect prone stock. Fifteen percent is normal for furniture and can reach thirty on wide boards you are cutting around knots. Construction lumber cut to length needs far less.
• Comparing a price per board foot with a price per linear foot. They are different units and the ratio depends on the cross section. A linear foot of 1×4 is a third of a board foot, a linear foot of 2×12 is two.

How this was checked

The engine was written twice and compared. The definition anchors it: 1 inch by 12 inches by 12 inches returns exactly 1 board foot, and the same board expressed as 1 by 12 by 1 foot returns 1 as well, so the two divisors of 144 and 12 agree. The three boards in the diagram were checked to be 12 board feet each by hand. A 2×4 at 8 feet returns 5.3333 billed and 3.5 actual, which are the two figures printed on the diagram. Every entry in the size table was checked against its own definition, so the loss column is computed from the ratio of the actual cross section to the billed one rather than being typed in.

Related Calculators and Reference

• Deck board calculator for decking, where the board length and joist spacing decide the order rather than the volume.
• Joist span calculator for working out what size the framing members need to be before you count them.
• Rafter calculator for roof members, where the lengths come off a pitch rather than a tape.
• Header sizing calculator for the built up members over openings, which is where nominal against actual thickness starts to matter structurally.
• Volume of 3D shapes calculator for turned or tapered stock that is not a plain rectangular section.
• Plywood Calculator: sheets of plywood or OSB for floors, roofs and walls.
• American Softwood Lumber Standard PS 20 the voluntary product standard that sets the actual sizes a nominal 2×4 must meet.