Table of Contents
What is the difference between Ra & Rz ?

- Ra – The arithmetic average of the absolute values of the profile height deviations from the centerline.
- Rz – the vertical distance from the highest peak to the lowest valley within the scanned profile.
You can learn more in our Surface Finish Units Dictionary.
How to Convert between Ra & Rz ?
A common question among machinists and engineers is, “How to convert from Ra to Rz?”. A precise conversion like one can make between Kw and HP is impossible since Ra and Rz are two different properties.
It is like asking, “how do I convert height to weight?”. You cannot convert. However, you can make a rough estimation based on statistics. A person with a height of 6 feet will weigh between 140 and 290 pounds (At a probability of 99%). We know he will not weigh 100 pounds, nor 400.
The same logic is applied when converting Ra to Rz (or Rz to Ra). Based on statistics, we can forecast the Rz range for a specific Ra with reasonable probability.
Ra / Rz Conversion formulas
Rz to Ra
Ra to Rz
Converting between Ra and Rz is not a good engineering practice. It is always recommended to Measure according to the method in which surface roughness is indicated on the drawing.
- As you can see in the below graphs, the inaccuracy grows in higher Ra /Rz values.
- The explanation is more clear when presented on the Log-Log graph.
Ra / Rz Convention Calculator
Ra / Rz Conversion Charts
Ra to Rz Metric
Ra [µm] | Rz Range [µm] |
---|---|
0.1 | 0.4 - 2.5 |
0.2 | 0.8 - 4 |
0.5 | 2 - 9 |
1 | 4 - 14 |
1.5 | 6 - 20 |
2 | 7 - 25 |
3 | 11 - 33 |
5 | 17 - 49 |
10 | 34 - 84 |
15 | 50 - 115 |
20 | 65 - 140 |
Ra to Rz Inch
Ra [µ Inch] | Rz Range [µ Inch] |
---|---|
4 | 16 - 100 |
8 | 32 - 160 |
20 | 80 - 360 |
40 | 160 - 560 |
60 | 240 - 800 |
80 | 280 - 1000 |
120 | 440 - 1320 |
200 | 680 - 1960 |
400 | 1360 - 3360 |
600 | 2000 - 4600 |
800 | 2600 - 5600 |
Rz to Ra Metric
Rz [µm] | Ra Range [µm] |
---|---|
1 | 0.03 - 0.2 |
5 | 0.3 - 1.3 |
10 | 0.6 - 3 |
20 | 1.5 - 6 |
50 | 5 - 15 |
100 | 13 - 32 |
150 | 21 - 49 |
Rz to Ra Inch
Rz [µ Inch] | Ra Range [µ Inch] |
---|---|
40 | 1 - 8 |
200 | 12 - 52 |
400 | 24 - 120 |
800 | 60 - 240 |
2000 | 200 - 600 |
4000 | 520 - 1280 |
6000 | 840 - 1960 |