Board feet is the standard unit for measuring lumber volume, and the formula is straightforward
Board feet measures the volume of lumber in a way that accounts for thickness, width, and length all at once. One board foot equals a piece of wood that is 12 inches long, 12 inches wide, and 1 inch thick. To find board feet, multiply the thickness in inches by the width in inches by the length in feet, then divide by 12. The formula is: (thickness × width × length in feet) ÷ 12 = board feet.
This measurement matters because lumber is priced and sold by the board foot, not by the piece. A 2×4 board that is 8 feet long costs more than a 2×4 that is 6 feet long, and the board foot calculation reflects that difference. Knowing how to compute board feet yourself prevents overpaying and helps you estimate material costs before you buy.
Key Takeaways
- Board feet = (thickness in inches × width in inches × length in feet) ÷ 12.
- Thickness and width are the actual dimensions of the lumber, not the nominal size printed on the label.
- Length must be converted to feet before you multiply; a 96-inch board is 8 feet.
- Multiply the board feet of one piece by the number of pieces to find total volume for an order.
- Hardwood is always sold by board feet; softwood (pine, fir) may be sold by the piece or by the foot depending on the supplier.
Understanding actual dimensions versus nominal size
Lumber labels show nominal dimensions, which are not the real measurements. A board labeled 2×4 is actually 1.5 inches thick and 3.5 inches wide. A 1×12 is actually 0.75 inches thick and 11.25 inches wide. These differences exist because lumber shrinks as it dries and is planed smooth after milling.
When you calculate board feet, use the actual dimensions, not the nominal ones. If you use the nominal 2×4 size in your formula, your calculation will be wrong. Measure the board yourself or ask the supplier for the actual thickness and width. Most suppliers will provide this information if you ask, and many list it on their website or in a catalog.
Length is usually accurate to the nominal size. A board labeled 8 feet is 8 feet long (or very close to it). You do not need to measure the length unless you are working with rough-sawn or reclaimed lumber, which may vary.
Step-by-step calculation for a single board
Start with a concrete example. You have a piece of pine that is labeled 2×6×10 (meaning 2 inches nominal thickness, 6 inches nominal width, 10 feet long). The actual dimensions are 1.5 inches thick and 5.5 inches wide.
Multiply: 1.5 × 5.5 × 10 = 82.5. Then divide by 12: 82.5 ÷ 12 = 6.875 board feet. That single board contains 6.875 board feet of lumber.
If you need 5 of these boards, multiply 6.875 × 5 = 34.375 board feet total. Round up to 35 board feet when you order, because suppliers usually sell in whole or half board feet.
Converting inches to feet before you calculate
Length must be in feet when you use the formula. If a board is 48 inches long, divide by 12 to get 4 feet. If a board is 96 inches long, that is 8 feet. Always convert length to feet first, or your answer will be off by a factor of 12.
Thickness and width stay in inches. Do not convert them to feet. The formula is built to work with thickness and width in inches and length in feet, then divide the result by 12 to get board feet.
A common mistake is converting everything to feet, which gives you cubic feet instead of board feet. Board feet and cubic feet are not the same. Lumber is always priced in board feet, so stick to the formula as written.
Using a board feet calculator or table
If you do not want to do the math by hand, a board feet calculator (available free online) will do it for you. Type in the actual thickness, width, and length, and it returns the board feet when ready. This is faster than hand calculation and removes the risk of arithmetic error.
Some suppliers also publish board feet tables that show common sizes. For example, a table might list that a 2×4×8 (actual 1.5×3.5×8) equals 4.67 board feet. If you are buying standard sizes, a table can be faster than a calculator.
Whether you calculate by hand, use a calculator, or use a table, the answer should be the same. If it is not, check that you are using actual dimensions, not nominal ones, and that length is in feet.
Hardwood versus softwood pricing
Hardwood (oak, maple, walnut) is always sold by the board foot. When you order hardwood, you specify the total board feet you need, and the supplier cuts pieces to your dimensions or sells you rough stock that you cut yourself.
Softwood (pine, fir, spruce) is sometimes sold by the piece and sometimes by the board foot, depending on the supplier and the grade. Home improvement stores often sell softwood by the piece, while lumber yards sell it by the board foot. Ask the supplier which method they use before you order.
If softwood is sold by the piece, you still need to know board feet to compare prices across suppliers. Calculate the board feet of one piece, multiply by the number of pieces you need, then divide the total price by the board feet to find the price per board foot. This lets you compare apples to apples.
Common sizes and their board feet
| Nominal Size | Actual Dimensions | Length | Board Feet |
|---|---|---|---|
| 1×4 | 0.75" × 3.5" | 8 feet | 1.87 |
| 1×6 | 0.75" × 5.5" | 8 feet | 2.75 |
| 1×12 | 0.75" × 11.25" | 8 feet | 5.63 |
| 2×4 | 1.5" × 3.5" | 8 feet | 4.67 |
| 2×6 | 1.5" × 5.5" | 8 feet | 6.88 |
| 2×8 | 1.5" × 7.25" | 8 feet | 9.33 |
| 2×10 | 1.5" × 9.25" | 8 feet | 11.88 |
| 2×12 | 1.5" × 11.25" | 8 feet | 15.00 |
These are standard softwood sizes. Hardwood is often sold in random widths and lengths, so you will need to calculate board feet for each piece individually. The table above is a reference for quick lookup if you are working with common softwood dimensions.
Frequently Asked Questions
What is the difference between board feet and linear feet?
Linear feet measures length only, not volume. A board that is 10 linear feet long could be 1 inch thick and 4 inches wide, or 2 inches thick and 12 inches wide — both are 10 linear feet, but they contain different amounts of wood. Board feet accounts for thickness and width, so it is the correct measure for pricing and material estimates.
Do I use nominal or actual dimensions when I calculate board feet?
Always use actual dimensions. A 2×4 is actually 1.5 inches by 3.5 inches. If you use the nominal 2 and 4, your calculation will be too high. Measure the board or ask the supplier for the actual thickness and width.
Why do I divide by 12 at the end?
The formula is built so that one board foot equals 144 cubic inches (12 × 12 × 1). When you multiply thickness × width × length in feet, you get a number that is 12 times too large, so you divide by 12 to convert it to board feet. This is just how the unit is defined.
Can I calculate board feet for plywood or other sheet goods?
Plywood is measured in sheets, not board feet, even though the formula would technically work. A sheet of plywood is priced as a sheet, and suppliers list the price per sheet, not per board foot. Stick with the sheet measurement for plywood.
What if the length is not a whole number of feet?
Convert the inches to a decimal. A board that is 7 feet 6 inches long is 7.5 feet. A board that is 9 feet 3 inches is 9.25 feet. Use the decimal in your formula just as you would a whole number.