Surface Finish Chart (Ra & N Grades)
Convert surface roughness between ISO 1302 N grades and Ra in micrometers and microinches, with the typical machining processes that reach each finish. Call out the Ra you need on the drawing — and pick a process that can hit it.
| N grade | Ra (µm) | Ra (µin) | Typical processes |
|---|---|---|---|
| N1 | 0.025 | 1 | Lapping, superfinishing |
| N2 | 0.05 | 2 | Lapping, honing |
| N3 | 0.1 | 4 | Honing, fine grinding |
| N4 | 0.2 | 8 | Grinding |
| N5 | 0.4 | 16 | Grinding, fine turning |
| N6 | 0.8 | 32 | Finish turning / milling, grinding |
| N7 | 1.6 | 63 | Turning, milling |
| N8 | 3.2 | 125 | Turning, milling, drilling |
| N9 | 6.3 | 250 | Rough turning / milling |
| N10 | 12.5 | 500 | Rough machining, sawing |
| N11 | 25 | 1000 | Rough / flame cutting |
| N12 | 50 | 2000 | Rough cutting, sand casting |
N-grade ↔ Ra per ISO 1302; µin are the conventional grade equivalents. The process column is approximate guidance — achievable finish depends on material, tooling, and setup.
Need a specific finish? Find finishing & grinding shops, or get the part quoted.
Get a quoteFrequently asked questions
- What is Ra?
- Ra is the arithmetic mean roughness — the average deviation of the surface profile from its mean line. It is the most common single-number surface-finish callout. Lower Ra means a smoother surface.
- What are N grades?
- ISO 1302 roughness grade numbers (N1–N12) are a shorthand for Ra values: N1 = 0.025 µm (smoothest) up to N12 = 50 µm (roughest). Each step roughly doubles the Ra.
- How do I convert Ra µm to µin?
- Multiply micrometers by ~39.37. The chart shows the conventional microinch grade equivalents (e.g., 0.8 µm ≈ 32 µin, 1.6 µm ≈ 63 µin, 3.2 µm ≈ 125 µin).
- What finish can a process achieve?
- As guidance: turning/milling typically reach Ra 1.6–6.3 µm (N7–N9), grinding 0.2–0.8 µm (N4–N6), and lapping/honing 0.025–0.1 µm (N1–N3). Achievable finish depends on material, tooling, and setup — call out the Ra you need rather than assuming a process.