Lumber Dimensions Chart
A 2×4 has not measured two by four inches since lumber started being sold surfaced. It leaves the mill at roughly that size and finishes at one and a half by three and a half after drying and planing. Every calculation that matters uses the actual number.
Generated from the same data the calculators use — a chart and a calculator on this site cannot disagree.
Nominal and actual dimensions, surfaced dry lumber
| Nominal | Actual (in) | Actual (mm) |
|---|---|---|
| 1×2 | 0.75" × 1.5" | 19 × 38 mm |
| 1×3 | 0.75" × 2.5" | 19 × 64 mm |
| 1×4 | 0.75" × 3.5" | 19 × 89 mm |
| 1×6 | 0.75" × 5.5" | 19 × 140 mm |
| 1×8 | 0.75" × 7.25" | 19 × 184 mm |
| 1×10 | 0.75" × 9.25" | 19 × 235 mm |
| 1×12 | 0.75" × 11.25" | 19 × 286 mm |
| 5/4×6 | 1" × 5.5" | 25 × 140 mm |
| 2×2 | 1.5" × 1.5" | 38 × 38 mm |
| 2×3 | 1.5" × 2.5" | 38 × 64 mm |
| 2×4 | 1.5" × 3.5" | 38 × 89 mm |
| 2×6 | 1.5" × 5.5" | 38 × 140 mm |
| 2×8 | 1.5" × 7.25" | 38 × 184 mm |
| 2×10 | 1.5" × 9.25" | 38 × 235 mm |
| 2×12 | 1.5" × 11.25" | 38 × 286 mm |
| 2×14 | 1.5" × 13.25" | 38 × 337 mm |
| 4×4 | 3.5" × 3.5" | 89 × 89 mm |
| 4×6 | 3.5" × 5.5" | 89 × 140 mm |
| 6×6 | 5.5" × 5.5" | 140 × 140 mm |
How to read this chart
The rule is straightforward: nominal dimensions under 2 inches lose half an inch, and dimensions of 8 inches and above lose three quarters. So 2×4 becomes 1.5×3.5, and 2×10 becomes 1.5×9.25.
Green or rough-sawn lumber is closer to its nominal size, which is why timber framing and some agricultural stock measure differently. Surfaced dry — the S4S material sold at any yard — follows the table above.
Board thickness in quarters is a separate convention: 5/4 means five quarters of an inch nominal, finishing at one inch. It is common in decking.
Engineered products do not follow these rules at all. LVL, LSL and I-joists are manufactured to their stated dimensions, so a 9.5 inch I-joist is 9.5 inches.
Sources
- American Softwood Lumber Standard for surfaced dry dimensions
Reference values for surfaced dry softwood lumber. Rough-sawn, green and engineered products differ.
Frequently asked questions
Why is a 2×4 not actually 2 by 4 inches?
It starts at roughly that size when rough sawn, then loses material to drying shrinkage and to being planed smooth on all four sides. The nominal name stuck to the pre-surfaced dimension.
What does 5/4 decking mean?
Five quarters of an inch nominal thickness, finishing at about one inch. It is thicker than a 1× board and thinner than a 2×, which makes it a common decking choice.
Are actual sizes the same everywhere?
For surfaced dry softwood in North America, yes — the dimensions are standardised. Rough-sawn, green and imported material can differ, and engineered lumber is made to its stated size.
Does this affect my calculations much?
Considerably. Using 6 inches instead of 5.5 for deck boards under-orders material by nine percent, which is enough to stop a job with rows left to lay.