A 2x4 measures 1 1/2 in x 3 1/2 in (38 x 89 mm), not 2 in x 4 in. The “2x4” name is nominal; the board is planed smaller after drying.
If you have ever bought a “2x4” and found it does not measure 2 inches by 4 inches, you are not holding a bad board. The name is a nominal size: a label from the sawmill, not the finished measurement. The number that matters when you cut and build is the actual size.
Why a 2x4 is not 2 inches by 4 inches
Lumber is first cut rough, then dried and planed smooth on all four faces. The drying and planing take material away, so the finished board ends up smaller than the rough piece it started as. The name stayed the same; the board did not. In the United States the finished sizes follow a common standard (the American Softwood Lumber Standard), which is why a 2x4 at one store matches a 2x4 at another.
Nominal vs. actual lumber sizes (chart)
These are the usual finished sizes of dimensional softwood lumber, in inches with millimeters alongside:
| Nominal name | Actual size (inches) | Actual size (mm) |
|---|---|---|
| 1x2 | 3/4 in x 1 1/2 in | 19 x 38 mm |
| 1x3 | 3/4 in x 2 1/2 in | 19 x 64 mm |
| 1x4 | 3/4 in x 3 1/2 in | 19 x 89 mm |
| 1x6 | 3/4 in x 5 1/2 in | 19 x 140 mm |
| 1x8 | 3/4 in x 7 1/4 in | 19 x 184 mm |
| 1x10 | 3/4 in x 9 1/4 in | 19 x 235 mm |
| 1x12 | 3/4 in x 11 1/4 in | 19 x 286 mm |
| 2x2 | 1 1/2 in x 1 1/2 in | 38 x 38 mm |
| 2x4 | 1 1/2 in x 3 1/2 in | 38 x 89 mm |
| 2x6 | 1 1/2 in x 5 1/2 in | 38 x 140 mm |
| 2x8 | 1 1/2 in x 7 1/4 in | 38 x 184 mm |
| 2x10 | 1 1/2 in x 9 1/4 in | 38 x 235 mm |
| 2x12 | 1 1/2 in x 11 1/4 in | 38 x 286 mm |
| 4x4 | 3 1/2 in x 3 1/2 in | 89 x 89 mm |
| 4x6 | 3 1/2 in x 5 1/2 in | 89 x 140 mm |
| 6x6 | 5 1/2 in x 5 1/2 in | 140 x 140 mm |
Two quick patterns help you remember it. For anything labeled 1x, the thickness is 3/4 in (19 mm). For anything labeled 2x, the thickness is 1 1/2 in (38 mm). Widths lose 1/2 in up to the 6-in size, and 3/4 in from the 8-in size up.
Why this matters when you build
- Write your cut list with actual sizes. A shelf that is “two 2x4s wide” is 7 in wide, not 8 in.
- Thickness adds up. Two 2x4s stacked flat make a 3 in thickness. Four 1x4s side by side cover 14 in, not 16 in.
- Joints change the math. When one board butts against the face of another, the length of the first board depends on the actual thickness of the second.
- Lengths are nominal too. An 8-foot board is usually a little longer than 96 in, but pre-cut wall studs are shorter, often 92 5/8 in. Measure the real board before you trust the label.
How to check a board before you cut
- Measure thickness and width with a tape measure or calipers at both ends and the middle. Boards vary slightly from piece to piece and with humidity.
- Sight along the edge to look for bows and twists. Choose the straightest boards for visible parts.
- Mark your cut line with a sharp pencil or a marking knife, and measure from the same end every time.
Frequently asked questions
What is the actual size of a 2x4?
A standard 2x4 measures 1 1/2 inches by 3 1/2 inches (38 x 89 mm).
What is the actual size of a 1x4?
A 1x4 measures 3/4 inch by 3 1/2 inches (19 x 89 mm).
Why is a 2x4 not 2 inches by 4 inches?
The name is a nominal size from when the board is rough-sawn. Drying and planing remove material, so the finished board is smaller.
Are 2x4s the same size everywhere?
In the United States, dimensional softwood lumber follows a common standard, so the actual sizes are the same from store to store. Always measure the board in your hands, because thickness can vary slightly.
What is the actual size of a 4x4?
A 4x4 measures 3 1/2 inches by 3 1/2 inches (89 x 89 mm).
Plans that already use the real sizes
Every Scribe & Square plan lists parts with actual lumber sizes, a shopping list by board length and the exact numbers to check as you build.
See the plans