Surface Roughness Chart (Ra, Rz, RMS, N-Grades)
Convert between Ra (μin and μm), Rz, RMS and ISO 1302 N-grades, with the machining process that typically delivers each finish.
| N grade | Ra (μin) | Ra (μm) | Rz (μin) | RMS (μin) | Typical process |
|---|
Standard: ISO 1302 / ISO 21920 / ASME Y14.36 · Data checked: 2026-10-14
Frequently asked questions
What is the difference between Ra and Rz?
Ra is the arithmetic average of the profile height over the sampling length, which makes it stable but insensitive to a single deep scratch. Rz is the maximum peak-to-valley height, so it catches the deep valleys that Ra averages away. A surface with a good Ra and a bad Rz can still leak.
What Ra should I specify?
The coarsest value that does the job. Ra 3.2 um (N8) is the usual as-machined default for non-critical faces, 1.6 um (N7) for general machined fits, and 0.8 um (N6) or better for sealing faces and bearing journals. Over-specifying surface finish raises cost sharply.
Why do drawings use N-grades that no longer exist?
N-grades came from ISO 1302:1992. The 2002 edition dropped them and ISO 1302 has since been replaced by ISO 21920-1:2021, so N-grades survive mainly on older drawings. ASME Y14.36 drawings call out Ra numerically instead. If you see an N-grade, convert it to Ra.
Is RMS the same as Ra?
No, though people use them interchangeably. RMS is the root-mean-square of the profile and is about 1.11 times Ra for typical machined surfaces. An old drawing calling out 63 RMS is roughly Ra 57 microinches, not Ra 63.