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Lumber sizes: nominal vs actual
What a board is called and what it measures: 1× and 2× dimensional lumber, posts, and the actual thickness of plywood, MDF and drywall, in inches and millimeters.
Plywood is usually a little under its nominal thickness; MDF and drywall are true to size. Measure the stock you have before you set a dado or a reveal.
Verify with your local code and AHJ. Reference only, no warranty.
Boards, nominal → actual
| Actual, in | mm | |
|---|---|---|
| 1×2 | 3/4 × 1 1/2 | 19 × 38 |
| 1×3 | 3/4 × 2 1/2 | 19 × 64 |
| 1×4 | 3/4 × 3 1/2 | 19 × 89 |
| 1×6 | 3/4 × 5 1/2 | 19 × 140 |
| 1×8 | 3/4 × 7 1/4 | 19 × 184 |
| 1×10 | 3/4 × 9 1/4 | 19 × 235 |
| 1×12 | 3/4 × 11 1/4 | 19 × 286 |
| 2×2 | 1 1/2 × 1 1/2 | 38 × 38 |
| 2×3 | 1 1/2 × 2 1/2 | 38 × 64 |
| 2×4 | 1 1/2 × 3 1/2 | 38 × 89 |
| 2×6 | 1 1/2 × 5 1/2 | 38 × 140 |
| 2×8 | 1 1/2 × 7 1/4 | 38 × 184 |
| 2×10 | 1 1/2 × 9 1/4 | 38 × 235 |
| 2×12 | 1 1/2 × 11 1/4 | 38 × 286 |
| 4×4 | 3 1/2 × 3 1/2 | 89 × 89 |
| 4×6 | 3 1/2 × 5 1/2 | 89 × 140 |
| 6×6 | 5 1/2 × 5 1/2 | 140 × 140 |
Plywood, nominal → actual
| Actual, in | mm | |
|---|---|---|
| 1/4 | varies — measure | — |
| 3/8 | 11/32 | 8.7 |
| 1/2 | 15/32 | 11.9 |
| 5/8 | 19/32 | 15.1 |
| 3/4 | 23/32 | 18.3 |
MDF (true to size)
| Actual, in | mm | |
|---|---|---|
| 1/4 | 1/4 | 6.3 |
| 1/2 | 1/2 | 12.7 |
| 5/8 | 5/8 | 15.9 |
| 3/4 | 3/4 | 19.0 |
Drywall (true to size)
| Actual, in | mm | |
|---|---|---|
| 1/4 | 1/4 | 6.3 |
| 3/8 | 3/8 | 9.5 |
| 1/2 | 1/2 | 12.7 |
| 5/8 | 5/8 | 15.9 |
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Questions
What are the actual dimensions of a 2×4?
1 1/2 × 3 1/2 inches (38 × 89 mm).
How thick is 3/4 plywood really?
Typically 23/32″ (18.3 mm). Measure yours.
What size is a 1×6 board?
A 1×6 actually measures 3/4 × 5 1/2 inches (19 × 140 mm).