How to Calculate Board Feet
A complete guide to the math behind lumber volume, with real examples you can follow along with.
The Formula
The formula for board feet is straightforward:
Multiply the three dimensions together and divide by 12. The result is the board footage. That is all there is to it.
A Simple Example
Say you have a board that is 1 inch thick, 6 inches wide, and 8 feet long. Punch the numbers in:
One board, 4 board feet. If your lumber yard charges $5 per board foot, that board costs $20.
Working with Nominal Dimensions
Here is where it gets tricky. If you buy a 2×6 from the home center, the label says 2×6, but the actual dimensions are 1.5 inches thick and 5.5 inches wide. The nominal size (the name) and the actual size (what you measure) are different.
For the board foot formula, you need the actual dimensions. A 10-foot 2×6 is:
If you used 2 and 6 instead of 1.5 and 5.5, you would get 10 board feet — an overestimate of about 45%. That is the difference between paying $55 and paying $80 for the same board at $8 per board foot.
Common Lumber Sizes Reference
| Nominal Size | Actual (in) | BF per 8′ | BF per 10′ |
|---|---|---|---|
| 1×4 | 0.75 × 3.5 | 1.75 | 2.19 |
| 1×6 | 0.75 × 5.5 | 2.75 | 3.44 |
| 2×4 | 1.5 × 3.5 | 3.5 | 4.38 |
| 2×6 | 1.5 × 5.5 | 5.5 | 6.88 |
| 2×8 | 1.5 × 7.25 | 7.25 | 9.06 |
| 4×4 | 3.5 × 3.5 | 8.17 | 10.21 |
Calculating Board Feet for a Whole Project
When you have multiple boards of different sizes, you calculate each one separately and add them up. This is where a calculator like the one on this site saves time. Instead of doing the math for every board, you enter each piece once, set the quantity, and the running total updates automatically.
For example, a simple bookshelf might need:
- Two 1×12 boards at 6 feet each for the shelves
- Two 1×4 boards at 8 feet each for the sides
- One 1×3 board at 6 feet for the back support
Each of these calculates differently because the dimensions are different. Add a waste factor of 10% and you know exactly how much lumber to buy.
Common Mistakes
Using nominal instead of actual. This is the most common error. Always use the actual dimensions after milling.
Forgetting to convert units. The formula expects thickness and width in inches, and length in feet. If you measure length in inches, divide by 12 first. If you use metric, the formula changes — use our calculator's metric mode instead.
Skipping the waste factor. Even professional builders waste 5-10% of their lumber. Beginners can waste 20% or more. Always add a buffer.
Try it yourself: Use the board foot calculator on this site to check your math and build a cut list for your next project.