Engineering Reference
Surface Finish Converter
Ra, RMS, and Rz across µm and µin, with the nearest ISO N grade and the process that typically produces it.
Equivalents
Ra
3.20 µm
126 µin
RMS (approx.)
140 µin
3.55 µm
Rz (approx.)
12.8 µm
typically 3 to 7 × Ra
Nearest ISO grade
N8
Ra 3.2 µm nominal
Typical process at this finish: standard machining (a common general-machining callout)
Ra ↔ µin is exact (× 39.37). RMS and Rz are standard approximations; different measurement definitions, not unit conversions.
ISO N grades at a glance
Each grade doubles the roughness of the one before it
| Grade | Ra (µm) | Ra (µin) | Typical process |
|---|---|---|---|
| N4 | 0.2 | 8 | honing, lapping |
| N5 | 0.4 | 16 | fine grinding |
| N6 | 0.8 | 32 | grinding, fine turning |
| N7 | 1.6 | 63 | finish turning, reaming |
| N8 | 3.2 | 125 | standard machining |
| N9 | 6.3 | 250 | rough machining |
| N10 | 12.5 | 500 | rough turning, sawing |
Finish symbols hide on every sheet of the drawing.
Mavlon reads surface finish callouts, general notes included, and carries each one into your AS9102 Form 3 as an inspectable characteristic.
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Frequently asked questions
How do you convert Ra to RMS?
RMS ≈ 1.11 × Ra for a typical machined surface. They are different statistical measures of the same profile, so the factor is an approximation, not a unit conversion. 3.2 µm Ra is roughly 140 µin RMS.
How do you convert Ra µm to µin?
Multiply by 39.37; this one is exact. 0.8 µm Ra = 31.5 µin Ra, 3.2 µm Ra = 126 µin Ra.
What is the relationship between Ra and Rz?
Rz is typically 3 to 7 × Ra depending on process, with 4 × as the common planning estimate. If the drawing specifies Rz, measure Rz; do not accept parts off the ratio.
What finish is standard for machined aerospace parts?
There is no universal default; 3.2 µm Ra (125 µin) is a common general-machining callout, 1.6 µm Ra often for located or sealing faces, 0.8 µm Ra and finer for bearing surfaces. The title block or general notes set the default; callouts override it.