Surface roughness Ra: N grades, processes and values to specify
Published September 28, 2026 — N1 to N12 grades with a µm / µin converter, what each process can hold, the Ra to specify for each function, and why Ra and Rz can't be converted.
Ra converter: µm, µin and N grades
Enter a value in any unit: the converter gives the equivalent in micrometres, in micro-inches (µin, US drawings) and the nearest N roughness grade.
The 12 N grades and their Ra values:
Values to remember: 3.2 for a “clean” machined surface, 1.6 for a fit, 0.8 for a precision seat. Each step in the series halves Ra… and roughly multiplies finishing time.
1. Which Ra each process can hold
Common Ra ranges (µm) in production. These are ballpark figures: tool, material, speeds and machine easily shift them. It's what a shop achieves without special effort.
Asking for 0.4 on a milled part means adding an operation (grinding or polishing). If that's not intended, it's over-specification you pay for on every part.
2. Which Ra to specify for each function
Ballpark values commonly used in general engineering. For bearings and seals, the manufacturer's documentation sets the exact value: it takes precedence.
Roughness must stay consistent with the tolerance: a dimension toleranced to a few micrometres on an Ra 6.3 surface can be neither made nor measured. The ISO 286 fit calculator shows the tolerances at stake.
3. Ra, Rz: the difference, and why you can’t convert
- Ra is the arithmetic mean deviation of the profile from its mean line. It's an average: it smooths out isolated defects.
- Rz is the maximum height of the profile (highest peak + deepest valley), averaged over the sampling lengths. It sees a scratch that Ra almost ignores.
- There is no exact conversion: the Rz/Ra ratio depends on the process. You often read “Rz ≈ 4 to 7 × Ra” for machining, but it's only a rough guide. If a customer's drawing is in Rz, measure in Rz.
On the standards side, surface texture specification has been overhauled: ISO 21920 (2021) replaces ISO 1302 for drawing indication and ISO 4287 for profile parameters. The symbols stay close, but some notation details change: check that your sheet format and SOLIDWORKS annotations follow the version your company has adopted.
4. On the drawing: one general roughness, then exceptions
As with ISO 2768 general tolerances, good practice is to state a general roughness near the title block (for example Ra 3.2), followed in brackets by the special values, then only mark the surfaces that differ. The drawing then shows at a glance where the functional surfaces are.
In SOLIDWORKS, surface finish symbols can be placed in the model (3D annotations) and imported into the drawing: they follow the face and stay correct as the geometry changes.
5. FAQ
What does Ra 3.2 mean?
That the arithmetic mean deviation of the surface profile from its mean line is at most 3.2 µm. It's grade N8, typical of a clean turned or milled surface with no sliding function.
What does grade N7 correspond to?
Ra 1.6 µm (about 63 µin). It's the usual value for clearance fits and carefully machined surfaces.
How do I convert Ra to Rz?
There is no exact conversion: the ratio depends on the process and the profile. In machining, Rz is often 4 to 7 times Ra, but this approximation must not be used to accept a part: measure the parameter required on the drawing.
What Ra does turning achieve?
Commonly 0.8 to 6.3 µm depending on feed, tool nose radius and material. With careful finishing, 0.8 to 1.6 µm; below that, you move to grinding.
How do I convert µm to µin?
Multiply by 39.37: Ra 0.8 µm ≈ 31.5 µin, Ra 1.6 µm ≈ 63 µin, Ra 3.2 µm ≈ 126 µin.