Reference chart
Lumber Size Chart: Nominal vs Actual
By &Pixels, the studio that builds MyCalculator.to. Figures are for surfaced dry softwood. Rough-sawn and hardwood are measured differently, and both are covered below.
What you ask for at the yard, and what you actually measure when you get it home. The gap is half an inch on every dimension and it accumulates.
Why is a 2x4 not 2 inches by 4 inches?
Because the name describes the board before it was finished. A 2x4 is sawn green at roughly two by four inches, then dried, which shrinks it, then planed smooth on all four faces, which takes more off. What arrives at the yard measures 1.5 by 3.5 inches.
The name outlived the process that made it true, and is now standardised as a name rather than corrected. This is only a nuisance until something is stacked: two 2x4s laid flat come to 3 inches, not 4, and across a framed wall those half inches add up to real error.
Softwood boards
| Nominal | Actual, inches | Actual, mm |
|---|---|---|
| 1 x 2 | 0.75 x 1.5 | 19 x 38 |
| 1 x 3 | 0.75 x 2.5 | 19 x 64 |
| 1 x 4 | 0.75 x 3.5 | 19 x 89 |
| 1 x 6 | 0.75 x 5.5 | 19 x 140 |
| 1 x 8 | 0.75 x 7.25 | 19 x 184 |
| 1 x 10 | 0.75 x 9.25 | 19 x 235 |
| 1 x 12 | 0.75 x 11.25 | 19 x 286 |
| 2 x 2 | 1.5 x 1.5 | 38 x 38 |
| 2 x 3 | 1.5 x 2.5 | 38 x 64 |
| 2 x 4 | 1.5 x 3.5 | 38 x 89 |
| 2 x 6 | 1.5 x 5.5 | 38 x 140 |
| 2 x 8 | 1.5 x 7.25 | 38 x 184 |
| 2 x 10 | 1.5 x 9.25 | 38 x 235 |
| 2 x 12 | 1.5 x 11.25 | 38 x 286 |
| 4 x 4 | 3.5 x 3.5 | 89 x 89 |
| 4 x 6 | 3.5 x 5.5 | 89 x 140 |
| 6 x 6 | 5.5 x 5.5 | 140 x 140 |
The pattern changes above 8 inches. Up to that point the loss is half an inch; from 1x8 and 2x8 upward it is three quarters, which is why a 2x10 is 9.25 rather than 9.5.
The calculators behind this
Sheet goods
| Material | Nominal | Actual |
|---|---|---|
| Plywood, 1/4 in | 0.25 in | 7/32 in, about 0.219 |
| Plywood, 1/2 in | 0.5 in | 15/32 in, about 0.469 |
| Plywood, 3/4 in | 0.75 in | 23/32 in, about 0.719 |
| OSB, 7/16 in | 7/16 in | sold at the actual figure |
| Drywall, 1/2 in | 0.5 in | a true 0.5 in |
| Sheet size | 4 x 8 ft | a true 48 x 96 in |
Working with the gap
- Buy by the nominal name. The yard, the plans and the price list all use it. Nobody asks for a 1.5 by 3.5.
- Measure and cut with the actual size. Half an inch missing on each dimension is the difference between a tight joint and a gap.
- Add the thicknesses when stacking. Two 2x4s laid flat are 3 inches, not 4. Three studs sandwiched are 4.5, not 6.
- Check plywood against its actual thickness. Three-quarter plywood is 23/32. In a dado cut for exactly 0.75 it will rattle.
- Treat rough-sawn and hardwood differently. Rough-sawn really is its nominal size, and hardwood is sold in quarters of an inch.
For a cut list rather than a size lookup, the lumber calculator works in board feet and actual dimensions, and the deck board calculator accounts for the gap between boards, which is the other half inch people forget.
Formula & Methodology
Did you know? The gap has widened over time. Early twentieth century 2x4s finished closer to 1 5/8 by 3 5/8, and the current 1.5 by 3.5 was standardised in 1964. A board bought in 1920 and one bought today carry the same name and are not the same size.
Sources
- U.S. Department of Commerce Voluntary Product Standard PS 20, American Softwood Lumber Standard
- American Wood Council, span and sizing references for dimensional lumber
- APA The Engineered Wood Association, panel thickness tolerances for plywood and OSB