Nominal vs actual lumber dimensions
A 2x8 measures 1.5" by 7.25". Nominal sizes are names, not measurements — and estimating on the name is the most common way a materials list comes out short.
| Nominal | Actual thickness | Actual width | Narrower by | Typical use |
|---|---|---|---|---|
| 1x4 | 0.75" | 3.5" | 0.5" | Decking |
| 1x6 | 0.75" | 5.5" | 0.5" | Decking |
| 5/4x6 | 1" | 5.5" | — | Decking |
| 2x4 | 1.5" | 3.5" | 0.5" | Posts |
| 2x6 | 1.5" | 5.5" | 0.5" | Decking or framing |
| 2x8 | 1.5" | 7.25" | 0.75" | Joists and beams |
| 2x10 | 1.5" | 9.25" | 0.75" | Joists and beams |
| 2x12 | 1.5" | 11.25" | 0.75" | Joists and beams |
| 4x4 | 3.5" | 3.5" | 0.5" | Posts |
| 6x6 | 5.5" | 5.5" | 0.5" | Posts |
Why the numbers do not match
Lumber is named at the size it was rough sawn. It is then dried and surfaced on all four sides, which removes roughly a quarter inch per face. A board sawn at a full two inches by eight leaves the mill at 1.5" by 7.25", and that is what arrives on site.
The shortfall is not constant. Boards up to 6" nominal lose half an inch of width; 8" and wider lose three quarters. That is why a 2x6 is 5.5" but a 2x8 is 7.25" rather than 7.5".
The 5/4 exception
5/4x6 decking is quoted in quarters of an inch of rough thickness — five quarters, or 1.25" sawn — and surfaces to a full 1" thick by 5.5" wide. It is the most common dedicated decking stock precisely because that 1" thickness spans 16" joist spacing while costing less than a 2x6.
Where this bites
Two places. First, coverage: if you calculate deck boards using 6" instead of 5.5", you under-order by roughly one row in every twelve. Second, framing depth: a deck framed with 2x8 joists sits 7.25" deep, not 8", which matters when you are working back from a door threshold to set your ledger height.
These are the numbers we calculate with
Every quantity our tools produce is built from the actual dimensions in this table, not the nominal labels. The decking board calculator applies them along with your board gap, and the framing calculator uses them for joist layout.