Reference & Charts
Surface Finish (Ra) Chart
The surface roughness reference: ISO grade numbers N1–N12 with their exact Ra in micrometres and microinches, the machining processes that produce each finish, and where each is used. Convert between Ra µm, Ra µin, and N grade at a glance.
12 entries · Last verified: 2026-07-10
Surface finish is most often specified as Ra — the arithmetic mean roughness. The ISO grade numbers (N1–N12) each correspond to an exact Ra value, and the same finish is quoted in micrometres (µm) or microinches (µin) depending on the drawing. As a rule of thumb, each grade step doubles Ra. Ra 3.2 µm (125 µin, N8) is the usual default for a general machined surface; smoother finishes need grinding, honing, or lapping and cost progressively more.
| Surface Finish | ISO grade | Ra(µm) | Ra(µin) | Typical process |
|---|---|---|---|---|
| Ra 0.025 µm (N1) | N1 | 0.025 µm | 1 µin | Lapping, superfinishing |
| Ra 0.05 µm (N2) | N2 | 0.05 µm | 2 µin | Lapping, honing |
| Ra 0.1 µm (N3) | N3 | 0.1 µm | 4 µin | Honing, polishing |
| Ra 0.2 µm (N4) | N4 | 0.2 µm | 8 µin | Fine grinding, honing |
| Ra 0.4 µm (N5) | N5 | 0.4 µm | 16 µin | Grinding, fine turning/boring |
| Ra 0.8 µm (N6) | N6 | 0.8 µm | 32 µin | Finish turning/milling, reaming, grinding |
| Ra 1.6 µm (N7) | N7 | 1.6 µm | 63 µin | Turning, milling, reaming |
| Ra 3.2 µm (N8) | N8 | 3.2 µm | 125 µin | Turning, milling, drilling |
| Ra 6.3 µm (N9) | N9 | 6.3 µm | 250 µin | Rough turning/milling |
| Ra 12.5 µm (N10) | N10 | 12.5 µm | 500 µin | Rough machining, coarse feeds |
| Ra 25 µm (N11) | N11 | 25 µm | 1000 µin | Sawing, rough milling |
| Ra 50 µm (N12) | N12 | 50 µm | 2000 µin | Flame cutting, sand casting, forging |
Frequently asked
- How do I convert Ra µm to microinches?
- Multiply Ra in micrometres by 39.37 to get microinches (1 µm ≈ 39.37 µin). The chart uses the conventional rounded values — e.g. 0.8 µm ≈ 32 µin, 1.6 µm ≈ 63 µin, 3.2 µm ≈ 125 µin.
- What surface finish should I put on a general machined part?
- Ra 3.2 µm (125 µin, grade N8) is the common default for turned or milled surfaces — achievable without extra operations. Call out finer finishes (1.6, 0.8 µm or below) only on surfaces that need them, such as seals, bearings, and sliding fits, because each step down adds cost.
- Is Ra the same as the maximum roughness?
- No. Ra is an average; Rz (or Rmax) captures peak-to-valley height and is always larger. If a print calls out Rz, don't treat it as interchangeable with Ra — they measure different things.
Sources
- ISO 1302 / ISO 468 — surface roughness grade numbers and Ra values
- Typical process-to-finish guidance (machining practice)