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.