Shop tools
Surface finish converter
A drawing might call out Ra 1.6, N7 or 63 µin for the same surface. This converts between all of them, so a callout in one notation can be checked against a gauge reading in another.
The reference below is free to read — no account, no signup. The tool itself is inside MechCircle, which is free to start.
ISO 1302 roughness grades
| Grade | Ra (µm) | Ra (µin) | RMS (µin, approx.) |
|---|---|---|---|
| N1 | 0.025 | 0.98 | 1.09 |
| N2 | 0.05 | 1.97 | 2.19 |
| N3 | 0.1 | 3.94 | 4.37 |
| N4 | 0.2 | 7.87 | 8.74 |
| N5 | 0.4 | 15.75 | 17.48 |
| N6 | 0.8 | 31.50 | 34.96 |
| N7 | 1.6 | 62.99 | 69.92 |
| N8 | 3.2 | 125.98 | 139.84 |
| N9 | 6.3 | 248.03 | 275.32 |
| N10 | 12.5 | 492.13 | 546.26 |
| N11 | 25 | 984.25 | 1092.52 |
| N12 | 50 | 1968.51 | 2185.04 |
Each grade is double the one below it. N9 is 6.3 µm and N10 is 12.5 µm — the two most often misquoted.
How to use it
Type into whichever field you have and the rest follow. Enter an Ra value and it also names the tightest N grade that value satisfies — which is not always the nearest one, because an N grade on a drawing is a maximum permitted roughness rather than a target.
What it does and does not cover
- The RMS conversion uses the ×1.11 factor, which assumes an idealised sinusoidal profile. Real machined surfaces run roughly ×1.1 to ×1.3 depending on the process, so treat RMS as indicative rather than a measurement.
- Rz is deliberately not offered. The Rz to Ra ratio varies from about 3× to 7× with the machining process, so any fixed conversion factor would be actively misleading.
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