Surface roughness Ra and Rz
Convert Ra and Rz into each other, match values to the roughness grades N1 to N12 and check which manufacturing process achieves your target roughness.
Converter Ra ↔ Rz
Rz ≈ 6.4 to 11.2 µm
Roughness grade: N7
Approximation using the empirical factor Rz ≈ 4 to 7 · Ra. An exact conversion does not exist; the actual ratio depends on the manufacturing process.
Roughness grades N1 to N12 per ISO 1302
| Grade | Ra max. in µm |
|---|---|
| N1 | 0.025 |
| N2 | 0.05 |
| N3 | 0.1 |
| N4 | 0.2 |
| N5 | 0.4 |
| N6 | 0.8 |
| N7 | 1.6 |
| N8 | 3.2 |
| N9 | 6.3 |
| N10 | 12.5 |
| N11 | 25 |
| N12 | 50 |
The N grades originate from the withdrawn ISO 1302 and are informative only today. New drawings state Ra or Rz directly per ISO 21920.
Achievable roughness by manufacturing process
Typical Ra range: 0.1 to 1.6 µm
Typical Rz range: 0.8 to 10 µm
Yellow marker: your input value as Ra range.
Suitable processes for your target value:
Guide values from common reference tables. Achievable values depend on tooling, cutting parameters and material.
Formulas and fundamentals
Ra is the arithmetic mean deviation of the profile, Rz the maximum height of the profile within a sampling length. Both parameters are defined in ISO 21920.
An exact conversion between Ra and Rz does not exist. In practice the empirical ratio Rz ≈ 4 to 7 · Ra is used; the actual ratio depends on the manufacturing process and the profile shape.
The roughness grades N1 to N12 per ISO 1302 assign a grade to each Ra limit, from N1 with Ra 0.025 µm up to N12 with Ra 50 µm.
Worked example
For a ground functional surface with Ra 0.8 (grade N6) the Rz value typically lies in the range of about 3.2 to 5.6 µm. The calculator shows the range, the matching N grade and suitable manufacturing processes such as grinding or broaching.
Frequently asked questions
What is the difference between Ra and Rz?
Ra averages the profile deviations over the evaluation length, Rz rates the maximum profile height within a sampling length. Rz therefore reacts much more sensitively to single scratches and peaks and is often more meaningful for function-critical surfaces.
Why is the conversion between Ra and Rz only an approximation?
Ra and Rz describe different properties of the same profile. The ratio of Rz to Ra depends on the profile shape and thus on the manufacturing process: it tends to be lower for turned surfaces and higher for ground ones. The common empirical range is Rz ≈ 4 to 7 · Ra; an exact formula does not exist.
What changed with ISO 21920 compared to the old ISO 4287?
ISO 21920 replaces ISO 4287 (parameter definitions) and ISO 1302 (drawing indication). It combines parameters, measurement conditions and drawing indication in one series of standards, redefines default measurement conditions and refines, among other things, how Rz is determined over the sampling lengths.
Are the roughness grades N1 to N12 still valid today?
The N grades originate from the withdrawn ISO 1302 and are no longer intended on new drawings, where Ra or Rz are stated directly with a numerical value. On legacy drawings and in shop-floor practice the N grades are still widespread; N7 corresponds to Ra 1.6 µm, for example.
How is Rz indicated on drawings?
Rz is written in the surface texture symbol with its value in µm, for example Rz 6.3. Per ISO 21920 the value is by default an upper limit with the 16 % rule; if no measured value may exceed the limit, the suffix max is added (Rz max 6.3).
Which manufacturing process achieves which roughness?
Rough turning is typically at Ra 6.3 to 25, finish turning and milling at Ra 0.8 to 6.3, drilling at Ra 1.6 to 12.5, broaching at Ra 0.4 to 3.2, grinding at Ra 0.1 to 1.6 and lapping or polishing below that.