Surface Roughness Conversion Chart & Typical Values
The ultimate reference for Ra to Rz, N-grade, and RMS conversions. Essential for engineers working with international drawings and diverse measurement standards.
1. Master Roughness Conversion Table
Use the table below to quickly convert between common surface roughness units and ISO N-grade classifications.
| Ra (µm) | Ra (µinch) | N-Grade | Rz (µm) | RMS (µm) |
|---|---|---|---|---|
| 0.0125 | 0.5 | N1 | 0.05 | 0.014 |
| 0.025 | 1.0 | N2 | 0.1 | 0.028 |
| 0.05 | 2.0 | N3 | 0.2 | 0.055 |
| 0.1 | 4.0 | N4 | 0.4 | 0.11 |
| 0.2 | 8.0 | N5 | 0.8 | 0.22 |
| 0.4 | 16.0 | N6 | 1.6 | 0.44 |
| 0.8 | 32.0 | N7 | 3.2 | 0.88 |
| 1.6 | 63.0 | N8 | 6.3 | 1.76 |
| 3.2 | 125.0 | N9 | 12.5 | 3.52 |
| 6.3 | 250.0 | N10 | 25.0 | 6.93 |
| 12.5 | 500.0 | N11 | 50.0 | 13.75 |
| 25.0 | 1000.0 | N12 | 100.0 | 27.5 |
Conversion Factors:
- • µm to µinch: Multiply µm by 39.37 (e.g., 0.8 µm ≈ 32 µinch).
- • Ra to RMS: Ra ≈ 0.9 × RMS (for most processes).
- • Ra to Rz: Rz ≈ 4 to 7 × Ra (process dependent).
2. Typical Surface Roughness by Process
A quick reference for what Ra values are achievable using different manufacturing methods.
| Manufacturing Process | Ra (µm) Range | Economic Value (µm) |
|---|---|---|
| Sand Casting | 6.3 - 25.0 | 12.5 |
| Investment Casting | 1.6 - 6.3 | 3.2 |
| Forging | 3.2 - 12.5 | 6.3 |
| Turning (Rough) | 1.6 - 6.3 | 3.2 |
| Turning (Finish) | 0.4 - 1.6 | 0.8 |
| Milling | 0.8 - 6.3 | 1.6 |
| Grinding | 0.1 - 0.8 | 0.4 |
| Honing / Lapping | 0.025 - 0.2 | 0.1 |
| Polishing | 0.01 - 0.1 | 0.05 |
3. Understanding N-Grade Standards
The ISO N-grade system is an older but still widely used method for specifying surface roughness. Each N-grade represents a specific Ra value, with N1 being the smoothest and N12 being the roughest. Modern engineering drawings prefer explicit Ra values, but N-grades are common on legacy drawings.