Nominal Lumber Size Calculator
Nominal stamps to dressed inches, mm, and area.
What it does
Boards lie about their size on purpose. The stamp names the rough green blank; surfacing and drying shave it before it reaches the rack. A 2x4 arrives at 1.5 by 3.5 inches, and spans must use those numbers.
The tool above maps nine common nominal sizes to dressed inches, rounded millimeters, and section area. One menu covers both unit systems the way converters do, so metric cut lists and imperial span tables stay in sync.
NIST publishes the American Softwood Lumber Standard PS 20 behind these sizes, and the USDA Forest Products Lab documents the surfacing behind them. Two-by stock loses about half an inch per face; one-by loses about a quarter. The millimeter column follows published rounded equivalents.
Buy and build by actual. Joist tables, bolt edge distances, and hanger seats all assume dressed depth. When posts pass 5 inches or hardwoods enter, switch to timber or hardwood rules instead.
How to use
- Pick the nominal size from the list.
- Read actual inches and millimeters.
- Read section area in square inches.
- Use actual depth in span tables.
- Check posts and timbers separately.
- Buy by actual, not by stamp.
Formula
- nominal
- Store stamp size, such as 2x4.
- actual in
- Dressed size in inches.
- actual mm
- Published rounded metric equivalent.
Values are dressed dry sizes from the American Softwood Lumber Standard: 2x stock dresses 1/2 in under nominal in both faces, 1x dresses 1/4 in. Millimeter figures are the published rounded equivalents.
Worked example
Example: nominal 2x4
Standard SPF 2x4 framing stud.
- Nominal choice is 2x4.
- Actual: 1.5 x 3.5 in after surfacing.
- Metric: 38 x 89 mm.
- Section: 1.5 x 3.5 = 5.25 in2.
- Shop and span tables use the actual numbers.
Reference table
| Nominal | Actual in | Actual mm |
|---|---|---|
| 1x4 | 0.75 x 3.5 | 19 x 89 |
| 1x6 | 0.75 x 5.5 | 19 x 140 |
| 1x8 | 0.75 x 7.25 | 19 x 184 |
| 2x4 | 1.5 x 3.5 | 38 x 89 |
| 2x6 | 1.5 x 5.5 | 38 x 140 |
| 2x8 | 1.5 x 7.25 | 38 x 184 |
| 2x10 | 1.5 x 9.25 | 38 x 235 |
| 2x12 | 1.5 x 11.25 | 38 x 286 |
| 4x4 | 3.5 x 3.5 | 89 x 89 |
Limits
- Softwood dimension only. Hardwoods and timbers differ.
- Dry dressed sizes; green or rough stock runs bigger.
- Area only. Strength needs grade and span tables.
- Millimeter figures are rounded standards.
Common mistakes
- Framing spans off the nominal stamp.
- Drilling hangers for full depth.
- Mixing green and dry sizes in one wall.
- Reading 4x4 as a full 4 inches.
- Converting mm back with extra precision.
Try it yourself
Quick checks. Answers sit tucked under each question.
Actual size of a 2x6?
Actual is 1.5 x 5.5 in (38 x 140 mm). Area is 8.25 in2.
Actual size of a 4x4 post?
Actual is 3.5 x 3.5 in (89 x 89 mm). Area is 12.25 in2.
Questions
Why is a 2x4 smaller than 2 by 4?
Surfacing. Rough-sawn green boards start full size, then drying and planing shave them to the dressed standard. A 2x4 leaves the mill at 1.5 by 3.5 inches, dry and smooth.
How do inches convert to millimeters?
Multiply inches by 25.4 and round to the published size: 1.5 in is 38 mm, 3.5 in is 89 mm. This page prints both so metric plans and imperial spans agree.
Does this cover oak boards too?
No. Hardwood boards sell by quarter thickness and random widths with their own rules. This table covers softwood dimension lumber under the PS 20 standard.
What about wider joists?
Yes for common framing: 2x6 dresses 1.5 by 5.5, 2x8 1.5 by 7.25, 2x10 1.5 by 9.25, 2x12 1.5 by 11.25. Widths shrink more than thickness in drying.
Is a 4x4 really 4 inches?
A 4x4 post dresses 3.5 by 3.5 inches. Timbers over 5 inches follow different rules with smaller reductions, so check timber tables for big posts and beams.
Why does section area matter?
Area feeds section math: stiffness grows with width times depth cubed. Use actual depth in span and load tables, never the nominal stamp.