How to Calculate Board Feet: Formula, Examples and Lumber Size Table

Board feet = thickness (in) × width (in) × length (ft) ÷ 12, because 1 board foot is 144 cubic inches. A 2×4×8 ft board is 5.33 board feet by nominal size and 3.5 by actual size.

The two answers exist because lumberyards price softwood by nominal size, while the board you carry is smaller. Which one to use depends on how you are being charged, so this guide shows both.

How do you calculate board feet?

1 board foot (BF) = 1 in × 12 in × 12 in = 144 in³ = 1/12 ft³
BF = thickness (in) × width (in) × length (ft) ÷ 12
   = thickness (in) × width (in) × length (in) ÷ 144
total BF   = pieces × BF per piece
cost       = total BF × price per BF

1. Measure thickness and width in inches and length in feet. If you measured the length in inches, divide by 144 at the end instead of 12.
2. Multiply thickness × width × length.
3. Divide by 12.
4. Multiply by the number of identical pieces.
5. Decide whether you are using nominal or actual dimensions (next section), and say which one on any order or quote.

A board foot is a volume, not a length or an area. It is the amount of wood in a board 1 in thick, 12 in wide and 12 in long, which is 144 cubic inches, or 1/12 of a cubic foot (1 ft³ is 1,728 in³ per NIST Handbook 44). That is why a 1 in thick board with an area of 1 sq ft is exactly 1 board foot, and a 2 in thick board with the same area is 2.

Should you use nominal or actual size?

A 2×4 is called a 2×4, but a dry, surfaced one measures 1-1/2 × 3-1/2 in. The American Softwood Lumber Standard (PS 20) lists nominal and minimum dressed sizes, and SPIB explains that the 2×4 and 2×6 are 1/2 in narrower than their names and the 2×8 to 2×12 are 3/4 in narrower. The two ways to count therefore disagree.

• Nominal basis. Multiply the size in the name. This is the convention many lumberyards use to price softwood dimension lumber, so use it when checking a price per board foot.
• Actual basis. Multiply the measured size. This is the real volume of wood, so use it when you need to know how much material you have.

Sources disagree on which is “the” board-foot rule for dimension lumber, so ask your supplier which they bill on. Hardwood is different: it is sold by thickness in quarters of an inch, covered in the hardwood section below.

Actual sizes are the dry, surfaced (S4S) sizes from PS 20 as compiled by Arch Toolbox, and they match SPIB’s text. Green lumber is surfaced larger because it shrinks as it dries, and a board can be up to 3 in longer than its nominal length, so use the length you order. The board-foot columns are arithmetic on those sizes.

Worked examples

• Twenty 2×4×8 studs. Nominal: 2 × 4 × 8 ÷ 12 = 5.3333 BF each, so 20 × 5.3333 = 106.67 BF. Actual: 1.5 × 3.5 × 8 ÷ 12 = 3.5 BF each, so 20 × 3.5 = 70 BF.
• One 1×6×10. Nominal: 1 × 6 × 10 ÷ 12 = 5 BF. Actual: 0.75 × 5.5 × 10 ÷ 12 = 3.4375 BF.
• One hardwood board, 4/4 × 7 in × 8 ft. 1 × 7 × 8 ÷ 12 = 4.667 BF.

Notice the 20 studs: the nominal count is 106.67 BF and the actual count is 70 BF, a difference of 36.67 BF. If a seller quotes by board foot, check which one the number is.

Quick table: board feet by length

Nominal-basis board feet for one piece. For actual-basis numbers use the formula with the sizes in the table above.

How are hardwood board feet counted?

Hardwood is sold in thickness quarters: 4/4 is 1 in, 5/4 is 1.25 in, 8/4 is 2 in, and so on. The thickness of the rough board goes into the formula, and your supplier tells you whether the price is for rough or surfaced stock. The table shows a board 6 in wide and 8 ft long.

Because board feet are volume, doubling the thickness doubles the number. For a purchase, measure each board and add the results, since widths and lengths vary. As arithmetic only, with a round price of $10 per board foot (not a market price), ten boards of 4/4 × 7 in × 8 ft are 10 × 4.667 = 46.67 BF, so 46.67 × $10 = $466.67.

What is the difference between a board foot and a linear foot?

A linear foot is length only: 12 inches along the board, whatever its section. A board foot is volume. Linear feet become board feet by multiplying by the section: BF = linear feet × thickness (in) × width (in) ÷ 12. Going the other way, linear feet = BF × 12 ÷ (thickness × width). For a nominal 2×4, 1 BF is 12 ÷ 8 = 1.5 linear feet; for a real 1-1/2 × 3-1/2 in section it is 12 ÷ 5.25 = 2.29 linear feet. The guide on how to calculate linear feet covers running length, and how to calculate square feet covers area. Framing a wall is a count of pieces, not a volume: see how many studs you need.

Assumptions and limits

• Dimensions are for dry, surfaced softwood dimension lumber from PS 20 as compiled by Arch Toolbox. Timbers, green lumber, rough-sawn and reclaimed stock differ, so measure them.
• This guide counts lumber you can measure. Estimating board feet in a standing tree or a log uses log rules that predict sawn yield, which is a different method, and it is not covered here.
• Board feet do not describe strength. Species, grade, spans and fasteners are structural questions for the lumber’s grade stamp, your building department or a licensed engineer.
• Price per board foot varies by supplier and species. Ask your supplier, and enter your own price in the calculator.

Common questions