4x6 actual size
3½ × 5½ inches
8.9 × 14.0 cm · 89 × 140 mm · sold as a 4x6
Nominal against actual
| Nominal | Actual | Difference | |
|---|---|---|---|
| Thickness | 4 in | 3½ in (89 mm) | −½ in |
| Width | 6 in | 5½ in (140 mm) | −½ in |
| Cross-section | 24 in² | 19.25 in² | −20% |
What a 4x6 is for
Heavier posts and short beams, and the header over a garage opening. Often sold as a treated post for ground contact.
The name is the rough-sawn size before drying and planing. Why every board is smaller than its name →
What this costs you in practice
The error compounds wherever boards sit side by side. Three 4x6s are 16½ inches across, not 18; eight of them are 44 inches rather than 48, which is 4 inches short over a run you might have measured on paper. That is why framing is dimensioned centre to centre and not edge to edge: 16 inches on centre is a real 16 inches whatever the boards themselves measure, so a sheet of plywood still lands on a stud.
Other 4-inch nominal boards
| Nominal | Actual | In mm |
|---|---|---|
| 4x4 | 3½ × 3½ in | 89 × 89 mm |
Frequently asked questions
What is the actual size of a 4x6?
3½ × 5½ inches — 8.9 × 14.0 cm, or 89 × 140 mm.
Why is a 4x6 not 4 by 6 inches?
Because it was, once. The board is sawn at roughly its nominal size while green, then dried and planed smooth on all four faces, and both processes take material off. The name records the rough-sawn size and the label never caught up. This is standardised, not accidental — every mill in North America surfaces to the same finished dimensions.
How much smaller is a 4x6 than its name?
½ inch off the thickness and ½ off the width, which is 20% less cross-section than the name implies. That matters for spans and for anything where you are counting boards to fill a width.
What lengths does a 4x6 come in?
Typically 6, 8, 10, 12, 14, 16, 20 feet. Unlike the cross-section, length is not reduced — an 8-foot board is 8 feet, or a little over, because it is cut to length after planing.