How to use this calculator
- Choose the known scale. Select Ra, Rz, RMS or ISO N grade.
- Enter the value. Enter the roughness value or the N grade.
- Read approximate equivalents. Read Ra, Rz, RMS and N grade in the active unit system.
- Check the drawing standard. Use the actual specified scale for inspection; converted values are only approximate.
How it works
Surface finish scales are not interchangeable measurements, so conversion is only approximate. For common machined surfaces this page uses: RMS ≈ 1.11 · Ra Rz ≈ 4 · Ra ISO N grade is treated as the nominal Ra sequence: Ra(µm) = 0.0125 · 2ᴺ
Worked example
Verified against the live calculator
A common machined finish of Ra 1.6 µm corresponds to about
Rz = 4 × 1.6 = 6.4 µm and
RMS = 1.11 × 1.6 = 1.78 µm. Because
1.6 = 0.0125 × 2⁷, the nominal ISO grade is N7.
Frequently asked questions
Can you convert Ra to Rz exactly?
No. Ra, Rz and RMS describe different properties of the same surface profile, so there is no exact universal conversion. This calculator uses common machining approximations: Rz ≈ 4·Ra and RMS ≈ 1.11·Ra.
What is ISO N roughness grade?
ISO N grades are nominal Ra bands. The common nominal sequence is N1 = 0.025 µm, N2 = 0.05 µm, doubling each grade up to N12 = 50 µm. The calculator uses Ra(µm) = 0.0125·2^N.
When should I use the surface finish calculator instead?
Use the surface finish calculator when you want theoretical turning finish from feed per revolution and nose radius. Use this converter when you already have one roughness scale and need an approximate equivalent.
Does this work in microinches?
Yes. Toggle Imperial and roughness values display in µin while the same internal Ra/Rz/RMS relationships are preserved.
Method & assumptions
- Rz/Ra ratio varies with manufacturing process and profile shape; 4× is a practical machining approximation.
- RMS/Ra ≈ 1.11 assumes a typical random profile; it is not exact for all surfaces.
- Use the roughness parameter specified on the drawing for inspection and acceptance.