A 5/4×6 is sold and labeled as 5/4×6, but it actually measures 1" × 5½". The difference comes from milling and drying — the board starts closer to its named size, then shrinks as it's surfaced smooth and dried to standard moisture content.
Standard deck boards — the most common decking size
Despite the name, 5/4 decking is about 1" thick, not 1¼" — the "5/4" refers to the rough-sawn thickness before drying and planing, the same nominal-vs-actual gap as dimensional lumber.
| Nominal Size | Actual Size | Actual (decimal) |
|---|