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