BuildCalc Guides · Lumber
What Is the Actual Size of a 2×4?
Every framer learns this on their first day. Every DIYer measures wrong on theirs. Here is the rule, the table for every dimensional lumber size, and why the labels lie.
The nominal-vs-actual rule
Lumber is sold by its nominal size — what the rough timber measured before processing. From rough log to lumberyard, dimensions shrink:
- Rough-cut: log sawn to nominal size (full 2 × 4 in).
- Drying: kiln-dried from ~25% moisture to ~19%. Shrinks ~1/8 inch each face.
- Surfacing: planed smooth on all 4 sides (S4S). Removes ~1/8 inch each face.
- Result: a 2 × 4 arrives at 1.5 × 3.5 inches — losing 1/2 inch on each dimension.
Dimensional lumber sizes
Every common size, nominal vs. actual. Use the right column for layout math.
| Nominal | Actual | Decimal in |
|---|---|---|
| 1 × 2 | 3/4 × 1-1/2 in | 0.75 × 1.5 |
| 1 × 3 | 3/4 × 2-1/2 in | 0.75 × 2.5 |
| 1 × 4 | 3/4 × 3-1/2 in | 0.75 × 3.5 |
| 1 × 6 | 3/4 × 5-1/2 in | 0.75 × 5.5 |
| 1 × 8 | 3/4 × 7-1/4 in | 0.75 × 7.25 |
| 1 × 10 | 3/4 × 9-1/4 in | 0.75 × 9.25 |
| 1 × 12 | 3/4 × 11-1/4 in | 0.75 × 11.25 |
| 2 × 2 | 1-1/2 × 1-1/2 in | 1.5 × 1.5 |
| 2 × 3 | 1-1/2 × 2-1/2 in | 1.5 × 2.5 |
| 2 × 4 | 1-1/2 × 3-1/2 in | 1.5 × 3.5 |
| 2 × 6 | 1-1/2 × 5-1/2 in | 1.5 × 5.5 |
| 2 × 8 | 1-1/2 × 7-1/4 in | 1.5 × 7.25 |
| 2 × 10 | 1-1/2 × 9-1/4 in | 1.5 × 9.25 |
| 2 × 12 | 1-1/2 × 11-1/4 in | 1.5 × 11.25 |
| 4 × 4 | 3-1/2 × 3-1/2 in | 3.5 × 3.5 |
| 4 × 6 | 3-1/2 × 5-1/2 in | 3.5 × 5.5 |
| 6 × 6 | 5-1/2 × 5-1/2 in | 5.5 × 5.5 |
| 8 × 8 | 7-1/4 × 7-1/4 in | 7.25 × 7.25 |
Standard S4S dimensional lumber per American Lumber Standard ALS PS 20. Rough-sawn timbers from specialty mills may run full nominal.
Why it matters
- Stud wall thickness. A 2 × 4 wall is 3.5 inches deep (stud) + 0.5 inch drywall both sides = 4.5 inches total finished wall.
- Joist spacing. A 16-inch on-center joist layout puts joist centers 16 inches apart, but the actual clear span between joists is 16 − 1.5 = 14.5 inches.
- Post hole math. A 4 × 4 post takes up 3.5 × 3.5 inches of hole, not 4 × 4. Affects concrete volume for fence and deck posts.
- Sheathing layouts. A 4 × 8 sheet of plywood spans exactly the 16 inch centers of 6 studs — that's a 96 inch sheet over studs at 0, 16, 32, 48, 64, 80 and 96 inches. The whole system is sized around the actual stud dimension.
Add and subtract real dimensions
BuildCalc's Quick Math adds and subtracts feet, inches and fractions — perfect for laying out studs, joists and trim using actual lumber dimensions instead of nominal.
Open Quick Math →FAQ
What is the actual size of a 2x4?+
A 2 × 4 is actually 1.5 inches by 3.5 inches. The "2 × 4" name refers to the nominal (rough-cut) size before the lumber is dried and surfaced — the actual finished dimensions are smaller.
Why is a 2x4 not actually 2 inches by 4 inches?+
Lumber is sold by nominal size, which is the rough-cut size before it dries and gets planed smooth on all 4 sides (S4S = "surfaced four sides"). Drying shrinks it ~1/4 inch, planing removes another ~1/4 inch — leaving a 2 × 4 at 1.5 × 3.5 inches. The American Lumber Standard ALS PS 20 has set these dimensions since 1924.
What is the actual size of a 2x6, 2x8, 2x10 and 2x12?+
2 × 6 is 1.5 × 5.5 in. 2 × 8 is 1.5 × 7.25 in. 2 × 10 is 1.5 × 9.25 in. 2 × 12 is 1.5 × 11.25 in. The thickness stays 1.5 inches across the whole 2x family — only the width changes. Notice the wider boards shrink more (~3/4 inch instead of 1/2) because the raw dimension was larger.
What is the actual length of an 8-foot 2x4?+
An 8 ft 2 × 4 is actually 8 feet long — or very close. Length is one dimension that is NOT planed shorter than nominal. You may see slight variation (1/4 inch over or under) depending on the mill, but it is essentially the labeled length.
What about 4x4 posts, are those actually 4 inches?+
No — a 4 × 4 is actually 3.5 × 3.5 inches. A 6 × 6 is 5.5 × 5.5 inches. An 8 × 8 is 7.25 × 7.25 inches. Same nominal-vs-actual rule. Rough-sawn timber-framing-grade 4x4s and 6x6s sold by specialty suppliers can be true 4 × 4 — but that is the exception.
Do plywood and OSB follow the same rule?+
No — plywood and OSB sheets are usually their stated size. A 4 × 8 sheet is actually 4 × 8 feet (48 × 96 inches). Thickness is the exception: a "1/2 inch" sheet is often 15/32 inch, a "3/4 inch" is 23/32 inch. The undersize is for cabinet-grade tolerance during assembly.
Related guides
- How many bags of concrete for a 4×4 post? — uses the actual 3.5 inch post width in the math.
- How many deck screws per board? — joist spacing for 16-inch OC framing.