Equivalent Ra values for Surface Finish Grade Numbers
Older drawings may use roughness grade numbers to indicate Ra values. The following table is given in ISO 1302:1992.
| Roughness N ISO Grade Numbers |
Roughness values Ra micrometers (µm) |
Roughness values Ra microinches (µin.) |
RMS | CLA (µin.) Center Line Avg. |
Rt Roughness, microns |
| N12 | 50 | 2000 | 2200 | 2000 | 200 |
| N11 | 25 | 1000 | 1100 | 1000 | 100 |
| N10 | 12.5 | 500 | 550 | 500 | 50 |
| N9 | 6.3 | 250 | 275 | 250 | 25 |
| N8 | 3.2 | 125 | 137.5 | 125 | 13 |
| N7 | 1.6 | 63 | 64.3 | 63 | 8.0 |
| N6 | 0.8 | 32 | 32.5 | 32 | 4.0 |
| N5 | 0.4 | 16 | 17.6 | 16 | 2.0 |
| N4 | 0.2 | 8 | 8.8 | 8 | 1.2 |
| N3 | 0.1 | 4 | 4.4 | 4 | 0.8 |
| N2 | 0.05 | 2 | 2.2 | 2 | 0.5 |
| N1 | 0.025 | 1 | 1.1 | 1 | 0.3 |