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Surface Finish Converter
Pick whatever the drawing gives you and read across. The column that matters most on a quote is the last one: on stainless, Ra 1.6 is free and Ra 0.4 is an operation.
- Ra
- -
- N grade
- -
- Ra, microinch
- -
- Legacy JIS mark
- -
- Rz, estimated
- -
- Typical process
- -
- Cost effect
- -
Rz is an estimate, not a conversion. Ra averages the profile and Rz measures peak to valley, so they describe different things and no exact relationship exists. Machined surfaces typically land between four and seven times Ra. Never convert one to the other on a drawing and treat the result as equivalent.
Reviewed 2026-09-23 against JIS B 0601, ISO 1302, ISO 21920, ISO 4287.
Full table
| N | Ra µm | Ra µin | Rz est. | Legacy mark | Typical process | Cost on stainless |
|---|---|---|---|---|---|---|
| N12 | 50 | 2000 | 200 to 350 | - | Rough cut, flame cut, as cast | Nothing to do |
| N11 | 25 | 1000 | 100 to 175 | One triangle | Rough turning or milling | This is what roughing leaves |
| N10 | 12.5 | 500 | 50 to 87.5 | - | Rough to medium machining | No extra work |
| N9 | 6.3 | 250 | 25.2 to 44.1 | Two triangles | Medium machining | No extra work |
| N8 | 3.2 | 125 | 12.8 to 22.4 | - | Normal finish pass | Standard machined finish |
| N7 | 1.6 | 63 | 6.4 to 11.2 | Three triangles | Good finish pass, sharp tooling | Specify this as your default. It costs nothing on stainless |
| N6 | 0.8 | 32 | 3.2 to 5.6 | - | Careful finish turning or milling | More attention, tooling changed on a schedule |
| N5 | 0.4 | 16 | 1.6 to 2.8 | - | Grinding, or very fine turning on a rigid machine | Usually a separate operation. This is where the cost step is |
| N4 | 0.2 | 8 | 0.8 to 1.4 | Four triangles | Grinding | Separate operation and separate fixturing |
| N3 | 0.1 | 4 | 0.4 to 0.7 | - | Fine grinding, honing | Outside process in many shops |
| N2 | 0.05 | 2 | 0.2 to 0.35 | - | Lapping, polishing | Outside process, own lead time and minimum lot |
| N1 | 0.025 | 1 | 0.1 to 0.175 | - | Lapping, electropolishing | Specialist. Confirm it is genuinely required |
Legacy JIS triangle marks
Withdrawn from the standard and still everywhere, because drawings outlive standards. Published conversion tables disagree on the four triangle value, which is exactly why the range is shown rather than a single figure.
| Mark | Meaning | Typical Ra, µm |
|---|---|---|
| Wavy line | No machining required, as supplied | - |
| One triangle | Rough finish | around 25 |
| Two triangles | Medium finish | around 6.3 |
| Three triangles | Fine finish | around 1.6 |
| Four triangles | Precision finish | around 0.2, some tables give 0.4 or 0.8 |
If a drawing carries triangle marks, get the intended Ra confirmed in writing before anyone prices a finish. The difference between three and four triangles is the difference between a finish pass and a grinding operation.
Two things Ra does not tell you
Lay. Ra says nothing about the direction of the tool marks. A shaft running in a lip seal wants a plunge ground or lapped surface with no helical lead, because a turned surface pumps fluid along the shaft. The Ra can be perfect and the seal will still leak. Use the lay symbol beside the finish mark.
Appearance. Roughness and cosmetic finish are related and not the same. If a face has to look a particular way, describe it and supply a boundary sample rather than tightening the number.
The full explanation is inthe surface finish guide, and what it does to a quote is in the cost guide.
Disagrees with the standard in front of you? Write to sales at suscnc.com with the clause and we will correct it and say so.