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Ra / Rz / RMS Roughness Conversion
Ra and Rz measure different things, so conversion is approximate and process-dependent.
Rule of thumb: Rz ≈ 4–7 × Ra depending on process (turning ~4×, milling ~5×, EDM ~7×). The table below shows commonly accepted equivalents.
Ra ↔ Rz ↔ RMS Conversion Table
| Ra (µm) | Ra (µin) | Rz (µm) | RMS (µin) | Typical process |
|---|---|---|---|---|
| 0.025 | 1 | 0.25 | 1.1 | Lapping / polishing |
| 0.05 | 2 | 0.5 | 2.2 | Lapping |
| 0.1 | 4 | 0.8 | 4.4 | Honing / fine grinding |
| 0.2 | 8 | 1.6 | 8.8 | Fine grinding |
| 0.4 | 16 | 3.2 | 17.6 | Grinding |
| 0.8 | 32 | 6.3 | 35.2 | Fine turning / boring |
| 1.6 | 63 | 12.5 | 70.4 | Turning / milling |
| 3.2 | 125 | 25 | 140 | Coarse machining |
| 6.3 | 250 | 50 | 280 | Rough milling |
| 12.5 | 500 | 100 | 560 | Rough turning / drilling |
FAQ
Is Ra to Rz conversion exact?
No. Ra is the average deviation, Rz the mean peak-to-valley height. The ratio depends on the process — treat tables as guidance.
What does Ra 3.2 mean?
An average roughness of 3.2 µm — a general machining finish, suitable for most non-sealing surfaces.