What is the actual size of a 2x4?
A 2x4 measures 1 1/2" by 3 1/2", not 2" by 4". Here is the dressed size of every common piece of softwood lumber, and the rule that produces them.
A 2x4 is actually 1 1/2" × 3 1/2".
It loses 1/2" of thickness and 1/2" of width to drying and surfacing — 5.25 in² of actual cross-section against the 8 in² the name implies.
How that is worked out
- The nominal name is the size the board was when it was sawn green and rough. It is a name, not a measurement, and it has been one since the 1920s.
- Drying shrinks it. Surfacing all four sides (S4S) planes it smaller again. What reaches the rack is the dressed size in the table below.
- The rule that reproduces the whole table: take 1/4" off a nominal thickness under 2", and 1/2" from 2" through 4". For width, take 1/2" off up to a nominal 6", and 3/4" from a nominal 8" up. The break is at eight inches, not six — a 1x6 is 5 1/2" and a 1x8 is 7 1/4".
Dressed sizes, dry and surfaced four sides
| Called | Actual thickness | Actual width | Cross-section |
|---|---|---|---|
| 1x2 | 3/4" | 1 1/2" | 1.13 in² |
| 1x3 | 3/4" | 2 1/2" | 1.88 in² |
| 1x4 | 3/4" | 3 1/2" | 2.63 in² |
| 1x6 | 3/4" | 5 1/2" | 4.13 in² |
| 1x8 | 3/4" | 7 1/4" | 5.44 in² |
| 1x10 | 3/4" | 9 1/4" | 6.94 in² |
| 1x12 | 3/4" | 11 1/4" | 8.44 in² |
| 5/4x6 | 1" | 5 1/2" | 5.50 in² |
| 2x2 | 1 1/2" | 1 1/2" | 2.25 in² |
| 2x3 | 1 1/2" | 2 1/2" | 3.75 in² |
| 2x4 | 1 1/2" | 3 1/2" | 5.25 in² |
| 2x6 | 1 1/2" | 5 1/2" | 8.25 in² |
| 2x8 | 1 1/2" | 7 1/4" | 10.88 in² |
| 2x10 | 1 1/2" | 9 1/4" | 13.88 in² |
| 2x12 | 1 1/2" | 11 1/4" | 16.88 in² |
| 4x4 | 3 1/2" | 3 1/2" | 12.25 in² |
| 4x6 | 3 1/2" | 5 1/2" | 19.25 in² |
Worth knowing
- These are the dry, surfaced sizes from US Department of Commerce PS 20, the American Softwood Lumber Standard.
- Green lumber is allowed to be slightly larger, because it has not finished shrinking. Rough-sawn stock is whatever the mill left it — measure it.
- Hardwood is not sold this way at all. It is sold rough by the quarter and priced by the board foot, which is a different arithmetic entirely.
- Plywood is the honest one: a sheet called 4x8 really is 48" × 96". Its thickness is not — nominal 3/4" ply is usually 23/32".
Work out your own numbers
- Board foot calculator
How much lumber to buy for a finished piece, what it will cost, and what to ask for at the yard.
- Board cut calculator
How many pieces of one length come out of a board of another, with the kerf charged between cuts and the leftover shown.
Related questions
- Why is a 2x4 not 2 inches by 4 inches?
- Because the name is the rough green size and the board is sold dry and surfaced. It was sawn at roughly 2" × 4", then it shrank as it dried and lost more to the planer that made all four faces flat and parallel. What is left is 1 1/2" × 3 1/2". The name stuck because the yard, the span tables and the framing trade all still talk in it.
- Was a 2x4 ever actually 2 inches by 4 inches?
- Roughly, yes — rough-sawn and green, before the standards of the 1920s and the revisions that followed settled on the dressed sizes we have now. Old houses genuinely do have fatter framing, which is why a 2x4 sistered onto a hundred-year-old stud does not sit flush.
- Is 5/4 decking really 1 1/4 inches thick?
- No — 5/4 ("five quarter") is a nominal 1 1/4" and dresses to a full 1". It is the usual softwood decking board, thicker than a 1x and cheaper and lighter than a 2x. Hardwood sold as 5/4 is a different thing again: there it means the rough thickness off the saw, and what you get after dressing depends on how straight it dried.