How to calculate board feet
A board foot is the unit lumber yards use to sell lumber by volume. The American Softwood Lumber Standard, Voluntary Product Standard PS 20-20 published by the National Institute of Standards and Technology (NIST), defines board measure in section 2.2: multiply the nominal thickness in inches by the nominal width in feet and by the length in feet. With the width in inches, as people usually say it, that becomes the formula below. A board 1 in thick, 12 in wide and 1 ft long is exactly one board foot.
Example: a 2×4 that is 8 ft long has 2 × 4 × 8 ÷ 12 = 5.33 board feet, and ten of them are 53.33 board feet. The length counts in actual feet, while thickness and width use the nominal size, the name of the board. That is also what the Wood Handbook of the USDA Forest Products Laboratory describes: lumber length is recorded in actual dimensions, width and thickness in nominal dimensions. When you order, make sure you and the seller count the same way, because the handbook points out that board measure depends on whether nominal or actual sizes are used.
| Size | 8 ft | 10 ft | 12 ft | 16 ft |
|---|---|---|---|---|
| 1×4 | 2.67 | 3.33 | 4 | 5.33 |
| 1×6 | 4 | 5 | 6 | 8 |
| 2×4 | 5.33 | 6.67 | 8 | 10.67 |
| 2×6 | 8 | 10 | 12 | 16 |
| 2×8 | 10.67 | 13.33 | 16 | 21.33 |
| 2×10 | 13.33 | 16.67 | 20 | 26.67 |
| 2×12 | 16 | 20 | 24 | 32 |
| 4×4 | 10.67 | 13.33 | 16 | 21.33 |
| 6×6 | 24 | 30 | 36 | 48 |
Nominal and actual lumber sizes
A 2×4 is not 2 by 4 inches. PS 20-20 sets a minimum dressed size for every nominal size, which is the size after the lumber has been surfaced, and the Wood Handbook (USDA Forest Products Laboratory, FPL-GTR-282) repeats the values for boards 1 in and thicker, dimension lumber and timbers in its Table 6-6. Dry lumber, dried to a moisture content of 19 percent or less, is dressed smaller than green lumber, because green lumber surfaced to the green size shrinks to about the dry size as it dries. The table lists the sizes from PS 20-20 Table 3 that the calculator uses.
| Nominal | Actual, dry | Dry in mm | Actual, green |
|---|---|---|---|
| 1×2 | 3/4 × 1 1/2 in | 19 × 38 mm | 25/32 × 1 9/16 in |
| 1×4 | 3/4 × 3 1/2 in | 19 × 89 mm | 25/32 × 3 9/16 in |
| 1×6 | 3/4 × 5 1/2 in | 19 × 140 mm | 25/32 × 5 5/8 in |
| 1×8 | 3/4 × 7 1/4 in | 19 × 184 mm | 25/32 × 7 1/2 in |
| 1×10 | 3/4 × 9 1/4 in | 19 × 235 mm | 25/32 × 9 1/2 in |
| 1×12 | 3/4 × 11 1/4 in | 19 × 286 mm | 25/32 × 11 1/2 in |
| 5/4×6 | 1 × 5 1/2 in | 25 × 140 mm | 1 1/32 × 5 5/8 in |
| 2×2 | 1 1/2 × 1 1/2 in | 38 × 38 mm | 1 9/16 × 1 9/16 in |
| 2×3 | 1 1/2 × 2 1/2 in | 38 × 64 mm | 1 9/16 × 2 9/16 in |
| 2×4 | 1 1/2 × 3 1/2 in | 38 × 89 mm | 1 9/16 × 3 9/16 in |
| 2×6 | 1 1/2 × 5 1/2 in | 38 × 140 mm | 1 9/16 × 5 5/8 in |
| 2×8 | 1 1/2 × 7 1/4 in | 38 × 184 mm | 1 9/16 × 7 1/2 in |
| 2×10 | 1 1/2 × 9 1/4 in | 38 × 235 mm | 1 9/16 × 9 1/2 in |
| 2×12 | 1 1/2 × 11 1/4 in | 38 × 286 mm | 1 9/16 × 11 1/2 in |
| 4×4 | 3 1/2 × 3 1/2 in | 89 × 89 mm | 3 9/16 × 3 9/16 in |
| 4×6 | 3 1/2 × 5 1/2 in | 89 × 140 mm | 3 9/16 × 5 5/8 in |
| 6×6 | 5 1/2 × 5 1/2 in | 140 × 140 mm | 5 1/2 × 5 1/2 in |
| 8×8 | 7 1/4 × 7 1/4 in | 184 × 184 mm | 7 1/2 × 7 1/2 in |
Boards (nominal thickness under 2 in) and dimension lumber (2 to 4 1/2 in) have their own rows in the standard. Timbers, 5 in and thicker, are dressed 1/2 in under nominal for 5 and 6 in, 3/4 in for 7 to 15 in and 1 in from 16 in when dry, and 1/2 in under nominal when green. A 6×6 is therefore 5 1/2 by 5 1/2 in either way. For a size the table does not list, the calculator still counts board feet, but leaves the piece out of the cubic meters.
Cubic meters and why board feet do not convert directly
Outside the US, lumber is sold by actual size in millimeters and by the cubic meter. PS 20-20 defines cubic measure from the dressed thickness and width times the length, so a cubic meter counts real wood, while a board foot counts the nominal size. That is why Appendix B1 of PS 20-20 asks for great care when converting board feet to cubic meters, and quotes the SI 10 note that the board foot is not a well-defined unit of volume. An 8 ft 2×4 is 5.33 board feet, which would be 768 cubic inches at its nominal size, but its dressed size of 1 1/2 by 3 1/2 in holds only 504 cubic inches, about 0.00826 m³.
In metric mode, enter lengths in meters and pick "Actual size in mm" for boards sold by their metric size, like 45 × 95 mm. Those pieces count toward the cubic meters, the linear meters and the cost, but not toward board feet, since they have no nominal size. Nominal sizes still work in metric mode, which helps when you compare a US plan with a local price list. The volume converter turns cubic meters into cubic feet or yards.
Cost and waste
Lumber is priced per piece, per linear foot or per board foot, depending on the seller and the product. Pick the one on your price list. For a price per piece, every line gets its own price field, because a 2×4 and a 2×12 cost different amounts. The cost is always worked out from the pieces you buy, after the waste allowance has been added and each line rounded up to whole boards. Hardwood boards for furniture are graded under the rules of the National Hardwood Lumber Association instead, so check how your supplier measures them before you compare prices.
Building a deck or a staircase? The stair calculator gives the stringer length and the number of treads, the fence calculator counts posts, rails and pickets, and the square footage calculator works out the area for decking.
How to use the lumber calculator
- 1Choose US feet or metric meters, then pick a size, the number of pieces and the length for the first line.
- 2Add a line for every other size or length. Use "Other nominal size" for sizes not in the list, or "Actual size in mm" for metric lumber.
- 3Choose a waste allowance and enter a price per board foot, per piece, per linear foot or per cubic meter.
- 4Read the board feet, linear feet, cubic meters and pieces to buy, then copy the order list or a link for the lumber yard.
Frequently asked questions
How do you calculate board feet?
Multiply the nominal thickness in inches by the nominal width in inches and the length in feet, then divide by 12. A 2×4 that is 8 ft long has 2 × 4 × 8 ÷ 12 = 5.33 board feet, and ten of them are 53.33 board feet. If you measure the length in inches, divide by 144 instead.
Do board feet use the nominal or the actual size?
For softwood lumber, the nominal size. The American Softwood Lumber Standard, PS 20-20, defines board measure as the nominal thickness in inches times the nominal width in feet times the length in feet. So a 2×4 counts as 2 by 4 in, even though it measures 1 1/2 by 3 1/2 in once dried and surfaced.
Why is a 2×4 not 2 by 4 inches?
The name is the nominal size. PS 20-20 sets a minimum dressed size for each nominal size, which is the size after the lumber has been surfaced. For a dry 2×4 that is 1 1/2 by 3 1/2 in (38 by 89 mm). Lumber surfaced green is a little larger, 1 9/16 by 3 9/16 in, and shrinks to about the dry size as it dries.
How many board feet are in a 2x4x8?
5.33 board feet: 2 × 4 × 8 ÷ 12. A 2×6×8 has 8, a 2×8×8 has 10.67, a 2×10×8 has 13.33 and a 2×12×8 has 16. Each extra 2 ft of length adds a quarter to an 8 ft board.
How do I convert board feet to cubic meters?
Not with a single factor. Board feet are counted from the nominal size, while PS 20-20 measures cubic meters from the dressed size, and the gap depends on the board. An 8 ft 2×4 is 5.33 board feet but only about 0.0083 cubic meters of actual wood. The calculator works out the cubic meters from the dressed size of each piece.
Should I add extra lumber for waste?
Usually some, for cutoffs, a split end or a badly warped board. There is no single standard number, so pick an allowance in the calculator. It adds the percentage to each line and rounds up to whole pieces, and the cost uses the pieces you actually buy.