Surface Finish Converter
Process-aware Ra ↔ Rz ↔ Rq ↔ Rt ↔ µin/µm with ISO 4288 cut-off and drawing callout.
Rz/Ra ratio used: 4.00
Ra (µm)
0.8
Ra (µin)
31.5
Rz (µm)
3.2
Rz (µin)
126
Rq / RMS (µm)
0.888
Rq / RMS (µin)
35.0
Rt (µm)
3.84
Rt (µin)
151
ISO 1302 N-grade
N6
≈ 0.8 µm Ra
ISO 4288 cut-off (λc)
0.8 mm
Sampling 0.8 mm · Evaluation 4 mm
ISO 2768 band
Fine (f)
General tolerance roughness class
Recommended drawing callout
Ra 0.8 / Rz 3.2 max, λc 0.8 mm
Per ISO 1302 — append / Gaussian filter, 5 sampling lengths if your QA group requires explicit filter convention.
Conversions: Rq ≈ Ra × 1.11, Rz ≈ Ra × ratio above, Rt ≈ Rz × 1.2. 1 µm = 39.37 µin. Rz/Ra ratios are process-dependent and only valid for stable, in-control processes.
ISO 1302 N-grade chart
- N10.025 µm
- N20.05 µm
- N30.1 µm
- N40.2 µm
- N50.4 µm
- N60.8 µm
- N71.6 µm
- N83.2 µm
- N96.3 µm
- N1012.5 µm
- N1125 µm
- N1250 µm
N-grades remain in informative use only — current ISO 1302 prefers explicit Ra / Rz values on drawings.
Typical Ra by process
- Lapping0.025 – 0.2 µm
- Polishing0.025 – 0.2 µm
- Grinding (precision)0.1 – 0.4 µm
- Honing0.1 – 0.8 µm
- Turning (finish)0.4 – 1.6 µm
- Milling (finish)0.8 – 3.2 µm
- EDM (finish)0.4 – 2.5 µm
- Drilling1.6 – 6.3 µm
- Broaching0.8 – 3.2 µm
- Casting (sand)12.5 – 25 µm
- FDM 3D print10 – 25 µm
- SLS 3D print5 – 15 µm
- SLA / DLP 3D print0.8 – 3.2 µm
ISO 4288 cut-off (λc) selection
| Ra range | λc (mm) | Eval. length (mm) |
|---|---|---|
| 0 – 0.02 µm | 0.08 | 0.4 |
| 0.02 – 0.1 µm | 0.25 | 1.25 |
| 0.1 – 2 µm | 0.8 | 4 |
| 2 – 10 µm | 2.5 | 12.5 |
| 10 – > µm | 8 | 40 |
For non-periodic profiles, ISO 4288. Periodic profiles use the RSm-based table — verify with your metrology lab.
Filter & profile notes
- FilterDefault per ISO 16610-21 is the Gaussian phase-correct filter. Older 2RC filters give 5–15% different results — never compare across filter types.
- ProfileR-profile (roughness) ≠ P-profile (primary, unfiltered) ≠ W-profile (waviness). Drawing callouts default to R-profile.
- StylusISO 3274 stylus tip 2 µm radius (typical). Larger tips smooth fine detail and under-report Ra.
- Rsk / RkuSkewness (asymmetry) and kurtosis (peakedness) are not interconvertible from Ra — they characterise wear and sealing behaviour separately.
