6x6 actual size
Dry (19% moisture or less)
5-1/2 × 5-1/2 in
139.7 × 139.7 mm
From the standard, NIST PS 20-25
Nominal, dry and green
| Size | Thickness | Width |
|---|---|---|
| Nominal (the name) | 6 in | 6 in |
| Dry, minimum | 5-1/2 in (139.7 mm) | 5-1/2 in (139.7 mm) |
| Green, minimum | 5-1/2 in (139.7 mm) | 5-1/2 in (139.7 mm) |
A 6x6 is timbers (5 in nominal thickness and up) under the American Softwood Lumber Standard, NIST PS 20-25. The standard names lumber by its nominal size and sets the smallest it may be once it has been dressed (planed smooth). Lumber dried to 19% moisture content or less is held to the dry size; lumber surfaced green, above 19%, is left a little bigger because it shrinks as it dries. The standard notes that timbers are often made and sold without drying, so expect the green size.
Board feet in a 6x6
Thickness × width ÷ 12 gives the board feet in each foot of length. By the nominal size that is 3 bd ft per foot; by the dry dressed size it is 2.521. Ask your yard which one it prices on.
| Length | Nominal bd ft | Actual (dry) bd ft |
|---|---|---|
| 8 ft | 24 | 20.17 |
| 10 ft | 30 | 25.21 |
| 12 ft | 36 | 30.25 |
| 16 ft | 48 | 40.33 |
For a whole order, use the board foot calculator. Every size side by side is on the lumber sizes chart.
Sources
- NIST (US Department of Commerce), Voluntary Product Standard PS 20-25, American Softwood Lumber Standard, Table 3 (effective January 2025), read 2026-11-07
- Engineering ToolBox, Softwood lumber: nominal and actual (dry) sizes, read 2026-11-07
- WoodBin, Standard dimensions of lumber (hardwood thickness after G. Wengert; softwood dry and green sizes), read 2026-11-07