How to Calculate Board Feet

The board foot formula

One board foot is 144 cubic inches of lumber, which is the same volume as a board 12 inches wide, 12 inches long, and 1 inch thick. The formula that gets you there from any board size is:

Board feet = thickness (inches) x width (inches) x length (feet) / 12

Notice the mixed units: thickness and width are in inches, but length is in feet. That is the part people trip on. If your length is in inches, divide it by 12 first, or use the all-inches version: thickness x width x length in inches, divided by 144. Both give the same answer.

Step by step

1. Measure the board. Use the nominal size, the size the lumber is sold as, not the actual size after planing. A nominal 2x4 measures 1.5 by 3.5 inches in your hand, but board feet are priced on the nominal 2 by 4.
2. Write down thickness and width in inches. For a 2x4, that is 2 and 4.
3. Write down length in feet. An 8 foot board is 8.
4. Multiply the three numbers, then divide by 12. That is the board footage of that one board.

Worked example 1: a single 2x4

A 2x4 that is 8 feet long: (2 x 4 x 8) / 12 = 64 / 12 = 5.33 board feet. If you want it in your head, note that a 2x4 contains 2/3 of a board foot per linear foot, so 8 feet is 8 x 2/3 = 5.33. A 2x6 is one full board foot per foot, which makes mental math easy: a 12 foot 2x6 is exactly 12 board feet.

Worked example 2: a 1x6 deck board, 8 feet long

(1 x 6 x 8) / 12 = 48 / 12 = 4 board feet. Notice how clean that is. One-inch-thick lumber has a shortcut: board feet equal width in inches times length in feet, divided by 12. A 1x8, 10 feet long is (1 x 8 x 10) / 12 = 6.67 board feet.

Worked example 3: a 4x4 fence post, 8 feet long

(4 x 4 x 8) / 12 = 128 / 12 = 10.67 board feet. Thick stock adds up fast, which is why posts and beams dominate a lumber bill.

Worked example 4: a whole cut list

Say you are building a small bookshelf and your cut list reads:

• 6 boards, 1x6, 8 feet: (1 x 6 x 8) / 12 = 4.00 board feet each, x 6 = 24.00
• 4 boards, 2x4, 8 feet: 5.33 board feet each, x 4 = 21.33
• 2 posts, 4x4, 6 feet: (4 x 4 x 6) / 12 = 8.00 board feet each, x 2 = 16.00

Add them: 24.00 + 21.33 + 16.00 = 61.33 board feet. That is the number you take to the lumberyard. Add 10 to 15 percent for waste before you buy, so order about 68 to 70 board feet.

Quick reference chart

Memorize the common ones and you will rarely need to calculate at all:

• 2x4 x 8 ft = 5.33 board feet
• 2x4 x 10 ft = 6.67 board feet
• 2x4 x 12 ft = 8.00 board feet
• 2x6 x 8 ft = 8.00 board feet
• 2x6 x 12 ft = 12.00 board feet
• 1x6 x 8 ft = 4.00 board feet
• 1x8 x 10 ft = 6.67 board feet
• 4x4 x 8 ft = 10.67 board feet

Nominal vs actual: which numbers go in the formula?

Use nominal sizes. A 2x4 is nominally 2 by 4 inches even though it actually measures 1.5 by 3.5 inches after drying and planing. The lumber industry prices board feet on nominal dimensions, so if you plug in the actual measurements you will undercount and underbudget. The one exception is when you are figuring out whether a board physically fits somewhere, like a shelf span. For fitting, use actual. For pricing, use nominal.

Common mistakes

Mixing up length units. Putting 96 (inches) into the length slot of the standard formula gives you an answer 12 times too big. Convert to feet first, or divide by 144 instead of 12 if everything is in inches.

Using actual dimensions for price math. As above, the yard bills on nominal. A 2x4 priced at 5.33 board feet per 8 foot stick is correct even though the stick is really 1.5 by 3.5 inches.

Forgetting waste. Saw kerf, end trimming, defects, and miscuts eat 10 to 15 percent of rough lumber. A 61.33 board foot cut list needs about 68 to 70 board feet purchased, and some yards will only sell whole boards, which rounds you up further.

Rounding each board. Round the total, not each line. Rounding six boards from 5.33 to 5.3 before multiplying can quietly erase a board foot or two from a large order.