Dimensional lumber is cut to a specific length, width, and depth. However, there is a difference between the nominal size (what the lumber is referred to) and the actual size.
Hardwood lumber is sized based on how many sides are surfaced. Surfaced refers to a finishing process that smoothes the lumber to remove all saw marks. If stock is surfaced green it is made larger than the dressed size of dry stock so that when it dries and shrinks it will be the same size as the dried stock.
In North America, hardwood lumber is usually specified using the quarter system, where a board is identified by its thickness in quarters of an inch. For instance, hardwood trim that ends up being just over 1" thick (surfaced on 2 sides) is called a five-quarter (5/4) board..
The tables below provide both the nominal and actual sizes so many common North American lumber sizes.
| NOMINAL SIZE (INCHES) | ACTUAL SIZE (INCHES) | ACTUAL SIZE (MM) |
| 1 x 1 - 1 x 12 | 3/4 x 1 ½ - 3/4 x 11 1/4 | 19 x 38 - 19 x 286 |
| 2 x 2 - 2 x 12 | 1 1/2 x 1 ½ - 1 1/2 x 11 1/4 | 38 x 38 - 38 x 286 |
| 3 x 3 - 3 x 12 | 2 1/2 x 2 ½ - 2 1/2 x 11 1/4 | 64 x 64 - 64 x 286 |
| 4 x 4 - 4 x 12 | 3 1/2 x 3 ½ - 3 1/2 x 11 1/4 | 89 x 89 - 89 x 286 |
| 6 x 6 | 5 1/2 x 5 1/2 | 140 x 140 |
| 8 x 8 | 7 1/4 x 7 1/4 | 184 x 184 |
| LENGTH IN FEET | LENGTH IN INCHES | LENGTH IN METERS |
| 6 feet - 24 feet | 72 inches - 288 inches | 1.829 meters - 7.315 meters |
| NOMINAL SIZE | ROUGH SIZE | ACTUAL SIZE: SURFACED 1 SIDE | ACTUAL SIZE: SURFACED 2 SIDES |
| - | 1/2 inch | 3/8 inch (9.5 mm) | 5/16 inch (7.9 mm) |
| - | 5/8 inch | 1/2 inch (13 mm) | 7/16 inch (11 mm) |
| - | 3/4 inch | 5/8 inch (16 mm) | 9/16 inch (14 mm) |
| 4/4 inch | 1 inch | 7/8 inch (22 mm) | 13/16 inch (21 mm) |
| 5/4 inch | 1 1/4 inch | 1 1/8 inch (29 mm) | 1 1/16 inch (27 mm) |
| 6/4 inch | 1 1/2 inch | 1 3/8 inch (35 mm) | 1 5/16 inch (33 mm) |
| 8/4 inch | 2 inches | 1 13/16 inch (46 mm) | 1 3/4 inch (44 mm) |
| 12/4 inch | 3 inches | 2 13/16 inch (71 mm) | 2 3/4 inch (70 mm) |
| 16/4 inch | 4 inches | 3 13/16 inch (97 mm) | 3 3/4 inch (95 mm) |