The actual measurable size of lumber (after milling, mosture changes, etc.) isn't the nominal size you see on the tags at Home Depot (or wherever). The following table shows what are more likely to be the actual real world dimensions.
| Nominal | Actual | Actual (Metric) |
|---|---|---|
| 1" x 2" | 3/4" x 1-1/2" | 19 x 38 mm |
| 1" x 3" | 3/4" x 2-1/2" | 19 x 64 mm |
| 1" x 4" | 3/4" x 3-1/2" | 19 x 89 mm |
| 1" x 5" | 3/4" x 4-1/2" | 19 x 114 mm |
| 1" x 6" | 3/4" x 5-1/2" | 19 x 140 mm |
| 1" x 7" | 3/4" x 6-1/4" | 19 x 159 mm |
| 1" x 8" | 3/4" x 7-1/4" | 19 x 184 mm |
| 1" x 10" | 3/4" x 9-1/4" | 19 x 235 mm |
| 1" x 12" | 3/4" x 11-1/4" | 19 x 286 mm |
| 1-1/4" x 4" | 1" x 3-1/2" | 25 x 89 mm |
| 1-1/4" x 6" | 1" x 5-1/2" | 25 x 140 mm |
| 1-1/4" x 8" | 1" x 7-1/4" | 25 x 184 mm |
| 1-1/4" x 10" | 1" x 9-1/4" | 25 x 235 mm |
| 1-1/4" x 12" | 1" x 11-1/4" | 25 x 286 mm |
| 1-1/2" x 4" | 1-1/4" x 3-1/2" | 32 x 89 mm |
| 1-1/2" x 6" | 1-1/4" x 5-1/2" | 32 x 140 mm |
| 1-1/2" x 8" | 1-1/4" x 7-1/4" | 32 x 184 mm |
| 1-1/2" x 10" | 1-1/4" x 9-1/4" | 32 x 235 mm |
| 1-1/2" x 12" | 1-1/4" x 11-1/4" | 32 x 286 mm |
| 2" x 4" | 1-1/2" x 3-1/2" | 38 x 89 mm |
| 2" x 6" | 1-1/2" x 5-1/2" | 38 x 140 mm |
| 2" x 8" | 1-1/2" x 7-1/4" | 38 x 184 mm |
| 2" x 10" | 1-1/2" x 9-1/4" | 38 x 235 mm |
| 2" x 12" | 1-1/2" x 11-1/4" | 38 x 286 mm |
| 3" x 6" | 2-1/2" x 5-1/2" | 64 x 140 mm |
| 4" x 4" | 3-1/2" x 3-1/2" | 89 x 89 mm |
| 4" x 6" | 3-1/2" x 5-1/2" | 89 x 140 mm |