The size on the tag is called the nominal size. It's a name more than a measurement. On framing lumber up to a 2×6, take 1/2″ off each number. From a 2×8 up, the wide face loses 3/4″ instead. Boards sold as 1× are 3/4″ thick. Plan around the actual numbers, because a shelf cut for a "4-inch" board will be 1/2″ off.
Lumber size chart
These are the standard surfaced sizes set by the American Softwood Lumber Standard, PS 20. Lengths are not reduced, so an 8-foot board is 8 feet long.
Boards
| Nominal | Actual |
|---|---|
| 1×2 | 3/4″ × 1 1/2″ |
| 1×3 | 3/4″ × 2 1/2″ |
| 1×4 | 3/4″ × 3 1/2″ |
| 1×6 | 3/4″ × 5 1/2″ |
| 1×8 | 3/4″ × 7 1/4″ |
| 1×10 | 3/4″ × 9 1/4″ |
| 1×12 | 3/4″ × 11 1/4″ |
Framing lumber
| Nominal | Actual |
|---|---|
| 2×2 | 1 1/2″ × 1 1/2″ |
| 2×3 | 1 1/2″ × 2 1/2″ |
| 2×4 | 1 1/2″ × 3 1/2″ |
| 2×6 | 1 1/2″ × 5 1/2″ |
| 2×8 | 1 1/2″ × 7 1/4″ |
| 2×10 | 1 1/2″ × 9 1/4″ |
| 2×12 | 1 1/2″ × 11 1/4″ |
Posts
| Nominal | Actual |
|---|---|
| 4×4 | 3 1/2″ × 3 1/2″ |
| 4×6 | 3 1/2″ × 5 1/2″ |
| 6×6 | 5 1/2″ × 5 1/2″ |
Why a 2x4 isn't 2 inches by 4 inches
A 2×4 starts life close to its name. It gets sawn at the mill, dried, then planed smooth on all four sides, and each step takes a little off. The standard fixes where it ends up, so a 2×4 from any mill matches a 2×4 from any other.
Wet lumber is the exception. Under PS 20, a 2×4 surfaced green is 1 9/16″ by 3 9/16″, because it still has shrinking to do. Treated lumber often arrives damp from treatment and shrinks a little as it dries. Old houses are another exception: their framing can be close to full size, so measure it before you buy.
Where the difference trips people up
- Stacked lumber. Three 2×4s on the flat are 4 1/2″ tall, and two 2×10s side by side are 3″ thick.
- Wall thickness. A 2×4 wall with 1/2″ drywall on both sides is 4 1/2″ thick. Use that number for door jambs and for working out how much room a new wall takes.
- Deck boards. A 5/4×6 is about 1″ thick and 5 1/2″ wide. Run that width through the deck calculator, and the gap is added for you.
- Plates in a wall. One bottom plate and two top plates add 4 1/2″ to the stud length. A 92 5/8″ precut stud makes a 97 1/8″ wall, tall enough for 8-foot drywall plus the 1/2″ drywall on the ceiling. The wall framing guide goes through the sum.
Hardwood goes by different rules
Hardwood is sold in quarters of an inch, measured rough. 4/4 is 1″ before planing and usually ends up around 13/16″ after. The yard still charges for the full inch, and widths are random. That's a different system from the chart above, and it's why rough hardwood is priced by the board foot.