Lumber sizes
Every common North American board size, nominal against actual, in inches and millimetres.
A 2x4 is not 2 by 4
It is 1½ × 3½ inches, and the same gap applies to every board on this page. The name records what the timber measured when it came off the saw, green and rough. Drying shrinks it, planing all four faces takes more off, and the label was never updated — so a board is sold under the size it used to be.
The rule is short enough to memorise:
| Nominal | Finishes at | Loses |
|---|---|---|
| 1 in | ¾ in | ¼ in |
| 2 to 6 in | nominal − ½ in | ½ in |
| 8 in and above | nominal − ¾ in | ¾ in |
Length is the exception. An 8-foot board really is 8 feet, often a fraction over, because it is cut to length after planing rather than before.
The full chart
| Sold as | Actual inches | Actual mm | Cross-section lost |
|---|---|---|---|
| 1x2 | ¾ × 1½ | 19 × 38 | 44% |
| 1x3 | ¾ × 2½ | 19 × 64 | 38% |
| 1x4 | ¾ × 3½ | 19 × 89 | 34% |
| 1x6 | ¾ × 5½ | 19 × 140 | 31% |
| 1x8 | ¾ × 7¼ | 19 × 184 | 32% |
| 1x10 | ¾ × 9¼ | 19 × 235 | 31% |
| 1x12 | ¾ × 11¼ | 19 × 286 | 30% |
| 2x2 | 1½ × 1½ | 38 × 38 | 44% |
| 2x3 | 1½ × 2½ | 38 × 64 | 38% |
| 2x4 | 1½ × 3½ | 38 × 89 | 34% |
| 2x6 | 1½ × 5½ | 38 × 140 | 31% |
| 2x8 | 1½ × 7¼ | 38 × 184 | 32% |
| 2x10 | 1½ × 9¼ | 38 × 235 | 31% |
| 2x12 | 1½ × 11¼ | 38 × 286 | 30% |
| 4x4 | 3½ × 3½ | 89 × 89 | 23% |
| 4x6 | 3½ × 5½ | 89 × 140 | 20% |
| 6x6 | 5½ × 5½ | 140 × 140 | 16% |
Standard lengths
Boards are stocked in even-numbered feet: 6, 8, 10, 12, 14, 16, 20. Longer lengths exist to order and cost disproportionately more, because the tree has to have been long enough.
Why it is worth checking before you cut
The error compounds where boards sit side by side. Three 2x6s are 16½ inches across, not 18 — an inch and a half short over three boards, and six inches short over a dozen. This is why framing is dimensioned centre to centre rather than edge to edge: 16 inches on centre stays 16 inches whatever the boards actually measure.
Where the missing half inch went
A 2x4 really was two inches by four, at the sawmill, while it was still green. It is then dried, which shrinks it, and planed smooth on all four faces, which takes more off. What comes out is what the American Softwood Lumber Standard fixes as the finished size, and the name it is sold under is the size it started at.
The rule has a step in it. Up to 6 inches nominal a board loses half an inch; from 8 inches up it loses three quarters. A 2x6 is 5½ inches wide and a 2x8 is 7¼, so the jump from 6 to 8 in the name is a jump of 1¾ inches in the wood — not the 2 inches the numbers suggest. Nominal 1-inch stock is the other exception, finishing at ¾.
Length is not shortened, only the cross-section
An 8-foot 2x4 is 96 inches long. The shrinkage applies to thickness and width and not to length, which is why you can buy studs cut to 92⅝ inches for a standard wall and get exactly that. It also means the only surprises are in the two dimensions you were not thinking about while measuring the run.
Span tables never use the nominal name
A joist's strength goes with the cube of its depth, so the difference between a nominal 10 and its actual 9¼ inches is not cosmetic — it is about 21% of the bending stiffness. Published span tables are written in actual dimensions for that reason, and a calculation done with the number on the label overstates what the timber will carry.
Plywood and sheet goods follow a different convention again: a sheet sold as ¾ inch is commonly 23/32, and one sold as ½ inch is 15/32. The shortfall is small and it is not covered by the rule above, so a ¾ sheet in a ¾ dado is a loose fit rather than a tight one.
Why nominal sizes are never the real size — lumber is one of six trades that do this. Unit converters · Paper sizes · Bed sizes