North American lumber is sized in inches, but if you're working from metric plans, ordering from a metric supplier, or just think in mm and cm, here's every common nominal size converted from its actual (not nominal) dimensions.
| Nominal Size | Actual (inches) | Metric (mm) |
|---|---|---|
| 1×2 | ¾" × 1½" | 19mm × 38mm |
| 1×4 | ¾" × 3½" | 19mm × 89mm |
| 1×6 | ¾" × 5½" | 19mm × 140mm |
| 1×8 | ¾" × 7¼" | 19mm × 184mm |
| 1×10 | ¾" × 9¼" | 19mm × 235mm |
| 1×12 | ¾" × 11¼" | 19mm × 286mm |
| 2×2 | 1½" × 1½" | 38mm × 38mm |
| 2×4 | 1½" × 3½" | 38mm × 89mm |
| 2×6 | 1½" × 5½" | 38mm × 140mm |
| 2×8 | 1½" × 7¼" | 38mm × 184mm |
| 2×10 | 1½" × 9¼" | 38mm × 235mm |
| 2×12 | 1½" × 11¼" | 38mm × 286mm |
| 4×4 | 3½" × 3½" | 89mm × 89mm |
| 6×6 | 5½" × 5½" | 140mm × 140mm |
If you convert the nominal name literally — 2 inches and 4 inches — you'd get 51mm × 102mm, which is wrong. The board you actually receive measures 1½" × 3½", which converts to 38mm × 89mm. This is the same nominal-vs-actual trap covered on our main explainer page — it just becomes more obvious once you're converting units, since the error compounds.
If you're building from architectural drawings or furniture plans that specify framing in millimeters, match against the metric column above rather than trying to reverse-engineer inches first. North American lumber yards will still sell and label everything by its nominal inch name (you'll ask for a "2x4," not a "38x89") — the metric conversion is just for your own planning and cut calculations.