What Board Feet Means and Why It Matters
Board feet is a standard unit of measurement for lumber volume. One board foot equals a piece of wood that is 12 inches long, 12 inches wide, and 1 inch thick. When you buy lumber by the board foot, you are paying for actual volume of wood, not just the length of a board. This matters because a 2×4 board that is 8 feet long contains more wood than a 2×4 that is 4 feet long, and the price reflects that difference.
Lumber yards and mills use board feet because it accounts for both the dimensions and the length of the wood. If you are building a deck, framing a wall, or ordering materials for any project, knowing how to calculate board feet helps you understand what you are actually buying and whether the price is fair. It also prevents you from ordering too much or too little material.
Key Takeaways
- Board feet is calculated by multiplying thickness (in inches) × width (in inches) × length (in feet), then dividing by 12.
- Nominal dimensions (what the board is called) are different from actual dimensions (what it really measures), and you must use actual dimensions in the formula.
- A 2×4 board that is 8 feet long contains 5.33 board feet, not 8 board feet.
- When ordering multiple boards, add up the board feet for each piece to get your total lumber volume.
The Board Feet Formula and How to Use It
The formula for board feet is straightforward: thickness (in inches) × width (in inches) × length (in feet) ÷ 12 = board feet. The key is that thickness and width must be in inches, but length must be in feet. Once you multiply those three numbers, you divide by 12 to convert to board feet.
Let's work through an example. A 2×4 board that is 8 feet long: multiply 2 × 4 × 8 = 64, then divide by 12. The result is 5.33 board feet. A 1×6 board that is 10 feet long: multiply 1 × 6 × 10 = 60, then divide by 12. The result is 5 board feet. Even though the second board is longer, both pieces contain roughly the same volume of wood.
Understanding Nominal vs. Actual Dimensions
Lumber is sold by its nominal size, which is what you call it when you order. A "2×4" is nominal. But the actual dimensions are smaller because of how the wood is milled and dried. A nominal 2×4 actually measures about 1.5 inches thick and 3.5 inches wide. A nominal 1×6 actually measures about 0.75 inches thick and 5.5 inches wide.
You must use the actual dimensions in your board feet calculation, not the nominal ones. If you use the nominal 2×4 dimensions (2 × 4 × 8 ÷ 12), you get 5.33 board feet. If you mistakenly use actual dimensions (1.5 × 3.5 × 8 ÷ 12), you get 3.5 board feet. The lumber yard uses actual dimensions, so your calculation must too. Ask your supplier for the actual dimensions of the lumber grade you are ordering, or look them up in a standard lumber sizing chart.
Calculating Board Feet for Different Lumber Sizes
Here are the actual dimensions for the most common nominal sizes:
| Nominal Size | Actual Dimensions | Board Feet (8 ft long) | Board Feet (10 ft long) | Board Feet (12 ft long) |
|---|---|---|---|---|
| 1×4 | 0.75" × 3.5" | 2.33 | 2.92 | 3.5 |
| 1×6 | 0.75" × 5.5" | 3.67 | 4.58 | 5.5 |
| 2×4 | 1.5" × 3.5" | 4.67 | 5.83 | 7 |
| 2×6 | 1.5" × 5.5" | 7 | 8.75 | 10.5 |
| 2×8 | 1.5" × 7.5" | 10 | 12.5 | 15 |
| 4×4 | 3.5" × 3.5" | 9.33 | 11.67 | 14 |
Use this table as a reference, but always verify the actual dimensions with your lumber supplier. Dimensions can vary slightly depending on the wood species, moisture content, and milling standards. If you are ordering a large quantity or an unusual size, ask the yard to confirm the actual measurements before you calculate.
Adding Up Board Feet for a Full Project
When you have multiple boards of different sizes, calculate the board feet for each one separately, then add them together. For example, if you are building a straightforward deck frame and need five 2×6 boards at 12 feet each and eight 2×4 boards at 8 feet each: five boards × 10.5 board feet per board = 52.5 board feet. Eight boards × 4.67 board feet per board = 37.36 board feet. Total: 52.5 + 37.36 = 89.86 board feet.
Round up slightly when you order, because you will likely have some waste from cutting, and a small amount of extra material is cheaper than a second trip to the lumber yard. In the example above, ordering 90 or 91 board feet is reasonable. Keep a record of what you ordered and what you actually used so you can estimate more accurately on future projects.
When to Ask a Lumber Yard for Help
If you have a detailed project plan with a materials list, many lumber yards will calculate board feet for you at no charge. Bring a sketch or a list of what you need, and the staff can tell you the total volume and the cost. This is especially useful if you are ordering specialty lumber or unusual sizes where you are not certain of the actual dimensions.
If you are ordering online or from a supplier that does not offer this service, do your own calculation and double-check it before you submit your order. A mistake in board feet can mean you receive too little material and have to reorder, or too much and waste money. Taking five minutes to verify the math is worth the cost savings.
Frequently Asked Questions
Do I use nominal or actual dimensions in the board feet formula?
Always use actual dimensions. A 2×4 is nominal; the actual size is 1.5 inches by 3.5 inches. Lumber yards measure and price based on actual dimensions, so your calculation must match theirs.
Why is a 2×4 board not 2 inches by 4 inches?
Lumber is milled to rough dimensions, then dried and planed smooth. The planing removes material, and drying shrinks the wood slightly. The result is smaller than the nominal size. This is standard across the industry.
How do I calculate board feet if the length is in inches instead of feet?
Convert inches to feet first by dividing by 12. A board that is 48 inches long is 4 feet long. Then use the formula: thickness × width × length in feet ÷ 12.
Can I use an online calculator instead of doing the math myself?
Yes. Many lumber suppliers and woodworking sites have board feet calculators where you enter the dimensions and it does the division for you. This is faster and reduces the chance of arithmetic errors, especially if you are calculating many boards.
What if I do not know the actual dimensions of the lumber I want to order?
Contact the supplier and ask for the actual dimensions of the specific grade and species you are ordering. Do not assume all 2×4s are the same size; hardwoods and softwoods can differ slightly, and different mills may have small variations.