How to calculate board feet

The formula

One board

thickness (in) × width (in) × length (ft) ÷ 12

All in inches

thickness × width × length (in) ÷ 144

Several boards

board feet of one × how many

Cost

board feet × price per board foot

Where the 12 and the 144 come from

The Wood Handbook defines the unit:

A unit of measurement of lumber represented by a board 12 in. long, 12 in. wide, and 1 in. thick or its cubic equivalent. [Wood Handbook, Glossary, p. G-2]

12 × 12 × 1 = 144 cubic inches, so a board’s board feet are its thickness × width × length in inches, divided by 144. Measure the length in feet instead and every foot is 12 inches, so the divisor becomes 12. The American Softwood Lumber Standard (PS 20) words it the same way, with the width in feet:

The number of board feet in a piece of lumber is obtained by multiplying the nominal thickness in inches or fraction of an inch by the nominal width in feet by the length in feet. [PS 20-20, section 2.2]

Step by step

An 8/4 board, 8 inches wide and 10 feet long. 8/4 is 2″ thick.

1. Multiply thickness, width and length in feet: 2 × 8 × 10 = 160.
2. Divide by 12: 160 ÷ 12 = 13.33 board feet.

The calculator shows the same working under every answer: 2 in × 8 in × 10 ft ÷ 12.

Use the nominal size, not the tape

The Wood Handbook adds how the count is made in practice:

In practice, the board foot calculation for lumber 1 in. or more in thickness is based on its nominal thickness and width and the actual length. [Wood Handbook, Glossary, p. G-2]

So the length is what the board measures, and the thickness and width are the nominal ones. A store 2x4, 8 feet long, counts as 2 × 4 × 8 ÷ 12 = 5.33 board feet. A tape on the dressed wood reads 1½ by 3½ and would give 3.50 board feet. The calculator counts the name by default and has a button for what you measured. For hardwood, type the thickness as it is called (4/4, 8/4) and the width you measure: hardwood has no nominal width. [Wood Handbook, ch. 6, p. 6-3] More on both: nominal vs actual sizes and what 4/4 means.

Under 1 inch

The same glossary entry has a second rule:

Lumber with a nominal thickness of less than 1 in. is calculated as 1 in. [Wood Handbook, Glossary, p. G-2]

A ½-inch board 12 inches wide and 8 feet long is 8 board feet, not the 4 its volume suggests. The calculator says so under the answer: Boards under 1 inch are priced as 1 inch thick, so this is 8 board feet, not 4.

Going backwards: how long a board to buy

Solve the formula for the length. To get 20 board feet from 1-inch stock 6 inches wide you need 20 × 12 ÷ (1 × 6) = 40 feet.

The cost

Multiply the board feet by the price per board foot. A 4/4 board, 6 inches wide and 8 feet long, is 4 board feet; at a price of $10.69 a board foot that is $42.76. The calculator rounds the money once, to the cent, from the unrounded board feet.

Waste

The list in the calculator can add a percentage on top of the total, in steps of 5%. Pick the percentage yourself; this page does not suggest one.