Actual dimensions of 6 x 6 lumber are 5-1/2″ x 5-1/2″ (139.7 x 139.7 mm). In fact, it’s smaller than what the nominal measurements indicate.
Nevertheless, hardware stores call it 6-by-6 lumber despite the incorrect nominal size due to the standard measurement.
However, technically the size of 6×6 is correct as it was measured before the lumber gets through a planer to create uniform surfaces. As a matter of fact, hardware stores sell lumber with uniform surfaces but their measurement is according to the original dimensions.
A 6×6 board has the same length regardless of its actual or nominal size. For example, an 8 feet long board will have an 8 feet actual length, the same is true with other sizes such as 6 feet, 10 feet, 4 feet, etc.
6×6 lumber dimensions
| 6×6 lumber | Nominal size in inches | Actual size in inches |
| 6 Width | 5-1/2 | |
| 6 height | 5-1/2 |
| 6×6 Lumber | Nominal size in mm | Actual size in mm |
| 25.4 | 19.05 | |
| 25.4 | 19.05 |
