Why lumber is smaller than its name
The name is the size of the board when it is first sawn from the log. The board is then dried, which shrinks it, and planed smooth on all four sides. What you buy is the dry, dressed size set by the American Softwood Lumber Standard (PS 20). The rule is simple: lumber 2 to 6 inches wide loses 1/2 inch, and lumber 8 inches and wider loses 3/4 inch. Length is not reduced: an 8 foot board is a full 8 feet.
2x dimension lumber (framing)
| Nominal | Actual size (inches) | Actual size (mm) | Board feet per linear foot |
|---|---|---|---|
| 2x2 | 1-1/2" x 1-1/2" | 38 x 38 mm | 0.333 |
| 2x3 | 1-1/2" x 2-1/2" | 38 x 64 mm | 0.5 |
| 2x4 | 1-1/2" x 3-1/2" | 38 x 89 mm | 0.667 |
| 2x6 | 1-1/2" x 5-1/2" | 38 x 140 mm | 1 |
| 2x8 | 1-1/2" x 7-1/4" | 38 x 184 mm | 1.333 |
| 2x10 | 1-1/2" x 9-1/4" | 38 x 235 mm | 1.667 |
| 2x12 | 1-1/2" x 11-1/4" | 38 x 286 mm | 2 |
1x boards (trim, shelving, fencing)
| Nominal | Actual size (inches) | Actual size (mm) | Board feet per linear foot |
|---|---|---|---|
| 1x2 | 3/4" x 1-1/2" | 19 x 38 mm | 0.167 |
| 1x3 | 3/4" x 2-1/2" | 19 x 64 mm | 0.25 |
| 1x4 | 3/4" x 3-1/2" | 19 x 89 mm | 0.333 |
| 1x6 | 3/4" x 5-1/2" | 19 x 140 mm | 0.5 |
| 1x8 | 3/4" x 7-1/4" | 19 x 184 mm | 0.667 |
| 1x10 | 3/4" x 9-1/4" | 19 x 235 mm | 0.833 |
| 1x12 | 3/4" x 11-1/4" | 19 x 286 mm | 1 |
Posts and timbers
| Nominal | Actual size (inches) | Actual size (mm) | Board feet per linear foot |
|---|---|---|---|
| 4x4 | 3-1/2" x 3-1/2" | 89 x 89 mm | 1.333 |
| 4x6 | 3-1/2" x 5-1/2" | 89 x 140 mm | 2 |
| 6x6 | 5-1/2" x 5-1/2" | 140 x 140 mm | 3 |
Other sizes you will see
- 5/4 decking: a 5/4x6 deck board is usually 1 inch thick and 5-1/2 inches wide.
- Precut studs: studs for 8 foot walls are commonly 92-5/8 inches long, so the wall reaches about 8 feet with plates. Use the stud calculator to count them.
- Rough hardwood is sold in quarters (4/4, 5/4, 8/4). A 4/4 board is 1 inch thick rough and about 13/16 inch after surfacing on two sides.
Plywood: nominal vs actual thickness
Sheathing-grade plywood is also slightly thinner than its label. Sanded cabinet plywood can differ again, so measure if the fit is tight.
| Nominal | Typical actual | Metric |
|---|---|---|
| 1/4" | 7/32" | 5.6 mm |
| 3/8" | 11/32" | 8.7 mm |
| 1/2" | 15/32" | 11.9 mm |
| 5/8" | 19/32" | 15.1 mm |
| 3/4" | 23/32" | 18.3 mm |
For sheet counts, use the plywood calculator.
Frequently asked questions
What is the actual size of a 2x4?
1-1/2 by 3-1/2 inches, or about 38 by 89 mm.
What is the actual size of a 2x6?
1-1/2 by 5-1/2 inches, or about 38 by 140 mm.
What is the actual size of a 4x4?
3-1/2 by 3-1/2 inches, or about 89 by 89 mm.
Why is a 2x4 not 2 inches by 4 inches?
Two inches by four inches is the rough sawn size. Drying and planing remove about 1/2 inch from each dimension.
Is the length of lumber also smaller?
No. Lengths are full size, so an 8 foot 2x4 is 8 feet long. Precut studs are the exception at about 92-5/8 inches.