Convert Ra to Rz
Convert roughness values in both directions, with the ISO 1302 roughness grade and the machining process that achieves it.
Ra to Rz conversion chart
Complete mapping of ISO 1302 roughness grades, from a sawn face to a polished surface. The Ra values of the N grades are standardised; the Rz correspondence is shop-floor convention and is therefore given as a range.
| Grade | Ra (µm) | Rz (µm) | Rz range | Process |
|---|---|---|---|---|
| N12 | 50 | 200 | 160 to 250 | Sawing, flame cutting |
| N11 | 25 | 125 | 100 to 160 | Roughing, rough turning |
| N10 | 12.5 | 63 | 50 to 80 | Roughing, rough turning |
| N9 | 6.3 | 25 | 20 to 40 | Turning and milling, rough |
| N8 | 3.2 | 12.5 | 10 to 20 | Standard CNC machining Usual standard |
| N7 | 1.6 | 6.3 | 5 to 10 | Finishing |
| N6 | 0.8 | 4 | 3.2 to 6.3 | Fine finishing |
| N5 | 0.4 | 2 | 1.6 to 3.2 | Grinding |
| N4 | 0.2 | 1 | 0.8 to 1.6 | Fine grinding |
| N3 | 0.1 | 0.63 | 0.4 to 1 | Lapping, honing |
| N2 | 0.05 | 0.32 | 0.25 to 0.4 | Polishing, superfinish |
| N1 | 0.025 | 0.16 | 0.1 to 0.25 | Polishing, superfinish |
Why there is no exact conversion
Ra is the arithmetic mean roughness. It averages all deviations of the profile from the mean line and therefore barely responds to individual deep grooves.
Rz is the mean roughness depth from the five highest peaks and the five deepest valleys. Rz is therefore sensitive to outliers.
Both parameters describe the same profile but weight it differently. A ground surface with many shallow grooves has a different Ra-to-Rz ratio than a turned surface with a regular feed profile. This is why neither ISO 21920 nor ISO 4287 provides a conversion. The rule of thumb Rz ≈ 4 to 6 × Ra is an approximation for the quoting stage, not a substitute for measurement.
Practical tip: Specify only the parameter on the drawing that matters for the function of the part. Stating both Ra and Rz on the same face forces two measurements and raises inspection cost without improving function. For sealing faces Rz is usually the more meaningful figure, for sliding and bearing surfaces Ra.
The roughness good shops achieve
Typical for CNC turning and milling without extra effort. Equals Rz 12.5, grade N8.
With an additional finishing pass on a machining centre. Equals Rz 4, grade N6.
By fine turning on a precise CNC lathe. Equals Rz 2, grade N5.
Surfaces finer than Ra 0.4 come from grinding, lapping or polishing. Send us the drawing and within 24 hours we tell you what makes economic sense and name the right partner shop.
Frequently asked questions
How do you convert Ra to Rz?
There is no exact conversion because Ra and Rz describe different properties of the same roughness profile. In practice the rule of thumb is Rz ≈ 4 to 6 × Ra. For machined surfaces Rz ≈ 4 × Ra is the usual figure. Example: Ra 3.2 corresponds to roughly Rz 12.5.
What is Ra 3.2 in Rz?
Ra 3.2 µm corresponds to approximately Rz 12.5 µm (practical range Rz 10 to 20). This is roughness grade N8 and at the same time the standard surface for CNC turning and milling without an additional finishing pass.
What is Rz 6.3 in Ra?
Rz 6.3 µm corresponds to approximately Ra 1.6 µm. This is roughness grade N7, achievable with a finishing pass at adjusted cutting data.
What does Ra mean?
Ra is the arithmetic mean roughness: the average of all deviations of the roughness profile from the mean line, measured over the evaluation length. Ra smooths out outliers and describes the average condition of a surface.
What is the difference between Ra and Rz?
Ra averages all profile deviations and therefore barely responds to individual deep grooves. Rz is the mean of the five highest peaks and the five deepest valleys and is therefore sensitive to outliers. Two surfaces with identical Ra can have markedly different Rz values. For sealing faces Rz is often the more meaningful figure.
Why can Ra not be converted exactly to Rz?
Because the ratio depends on the profile shape. A ground surface with many shallow grooves has a different Ra-to-Rz ratio than a turned surface with a regular feed profile. This is why ISO 21920 and ISO 4287 do not provide a conversion. If both values are relevant to the drawing, both must be measured.
What roughness is usual in CNC turning and milling?
The standard in machining is Ra 3.2 µm (Rz approx. 12.5), which is grade N8 without extra effort. Milling with a finishing pass reaches Ra 0.8 µm, fine turning Ra 0.4 µm. Finer surfaces come from grinding or polishing. State the finish in your enquiry and we place it with partner shops that deliver it.
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