Hardwood lumber thickness
Hardwood is sold rough, by the board foot, with its thickness written in quarters of an inch: 4/4 is 1 inch, 8/4 is 2 inches. The grading rules of the National Hardwood Lumber Association set the standard thickness each one comes to when it is surfaced on both faces (S2S).
From the standard, NHLA Rules (2019)
| Quarters | Rough | Surfaced (S2S) | S2S (mm) |
|---|---|---|---|
| — | 3/8 in | 3/16 in | 4.8 |
| — | 1/2 in | 5/16 in | 7.9 |
| — | 5/8 in | 7/16 in | 11.1 |
| — | 3/4 in | 9/16 in | 14.3 |
| 4/4 | 1 in | 13/16 in | 20.6 |
| 5/4 | 1-1/4 in | 1-1/16 in | 27 |
| 6/4 | 1-1/2 in | 1-5/16 in | 33.3 |
| 7/4 | 1-3/4 in | 1-1/2 in | 38.1 |
| 8/4 | 2 in | 1-3/4 in | 44.5 |
| 10/4 | 2-1/2 in | 2-1/4 in | 57.2 |
| 12/4 | 3 in | 2-3/4 in | 69.9 |
| 14/4 | 3-1/2 in | 3-1/4 in | 82.6 |
| 16/4 | 4 in | 3-3/4 in | 95.3 |
The rule behind the table
The surfaced thickness is the rough thickness less 3/16 inch for lumber up to 1-1/2 inches rough, and less 1/4 inch from 1-3/4 to 4 inches. Above 4 inches (16/4), the rules give no standard surfaced thickness; the rules also name 18/4 to 24/4 as rough thicknesses. A board surfaced on one side only has no standard size: that is agreed between buyer and seller.
You are billed for the rough size. The rules measure board feet before surfacing, and anything under 1 inch thick is counted as 1 inch, which is why the board foot calculator has a lumberyard tally mode and a “surfaced” option.
Edges have their own allowance: lumber sold at a specified width with its edges dressed may be 3/8 inch under the nominal width below 8 inches, and 1/2 inch under at 8 inches and wider.
Softwood construction lumber follows a different standard and different sizes: see the lumber sizes chart.