Quick Answer
4×4 Actual Size: 3.5 in × 3.5 in (89 mm × 89 mm)
The real, finished size of a standard “4×4” post is 3.5 inches by 3.5 inches, which is 89 mm by 89 mm.
“4×4” is the nominal name; after drying and planing, the board finishes smaller.
Quick dimensions
| Label | Inches | Millimeters |
|---|---|---|
| 4×4 actual size | 3.5″ × 3.5″ | 89 × 89 |
| 4×4 width | 3.5″ | 89 |
| 4×4 thickness | 3.5″ | 89 |
Also searched as: 4×4 dimensions, 4×4 actual measurement, 4×4 true size, 4×4 size in mm.
Nominal vs actual (why sizes are smaller)
“4×4” is the rough-cut (nominal) size. After the post is dried and planed smooth, its finished size becomes 3½″ × 3½″. This is the standard for North American softwood posts.
Compare to other common sizes
| Nominal | Actual (in) | Actual (mm) |
|---|---|---|
| 2×4 | 1.5 × 3.5 | 38 × 89 |
| 2×6 | 1.5 × 5.5 | 38 × 140 |
| 2×8 | 1.5 × 7.25 | 38 × 184 |
| 2×10 | 1.5 × 9.25 | 38 × 235 |
| 4×4 | 3.5 × 3.5 | 89 × 89 |
FAQ: 4×4 actual size
What is the actual size of a 4×4?
It’s 3.5 inches by 3.5 inches (89 mm × 89 mm).
Is a 4×4 really square?
Yes. Standard 4×4 posts are square in cross-section, finished to 3.5″ × 3.5″.
Do treated 4×4s measure the same?
Pressure-treated 4×4s are typically the same finished size (3.5″ × 3.5″). Minor variation can occur due to moisture content and milling.