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Ra – Rz – N Surface Roughness Converter
Do you have Ra, Rz or N roughness value and need to get its equivalent in other units? Use our free tool for that.
Please Note:
- A precise conversion between Ra and Rz values is impossible since they are two different properties. It is only possible to make a rough estimation based on statistics. The higher the roughness values the less precise is the conversion.
- Converting between Ra and Rz is not a good engineering practice. It is recommended to measure according to the method in which surface roughness is indicated on technical drawings.
FAQ on differences between Ra - Rz - N roughness measurements
How is surface roughness measured?
Both Ra and Rz are typically measured using profilometers. These devices can be contact-based, where a stylus physically traces the surface, or non-contact, using optical or laser methods. Non-contact methods are preferred for delicate or high-precision surfaces as they avoid physical interference with the surface being measured.
Ra (average roughness) is the predominant parameter for measuring surface roughness due to its ease of use and broad applicability. This simplicity makes it ideal for various applications, such as ensuring the smoothness of automotive parts, medical devices, and optical components.
While less common in modern practices, Rz (mean roughness depth) can be still found, especially in older technical drawings. This makes Rz particularly valuable for applications where extreme height variations impact performance, such as in sealing surfaces or components involved in sliding contacts.
What is the difference between Ra, Rz and N?
Ra (Arithmetic Mean Roughness)
Ra is the numerical average of the absolute values of the deviations of all the peaks and valleys from the mean line within the measurement length. It is also called the Center Line Average (CLA).
Rz (Mean Roughness depth)
Rz is the average distance between the highest peaks and lowest valleys (usually for five largest distances):
- From the highest peak to the lowest valley
- From the second highest peak to the second lowest valley
- etc. until the fifth’s biggest distance
N – Roughness Grade Numbers (DIN ISO 1302)
Older drawings may use roughness grade numbers according to DIN ISO 1302. This standard divides the surface roughness into twelve grades. Each class from N1 to N2 represents a maximum permissible Ra value according to this standard.
| DIN ISO 1302:1992 | N1 | N2 | N3 | N4 | N5 | N6 | N7 | N8 | N9 | N10 | N11 | N12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ra (µm) | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | 6.3 | 12.5 | 25 | 50 |
| Ra (µin.) | 1 | 2 | 4 | 8 | 16 | 32 | 63 | 125 | 250 | 500 | 1000 | 2000 |
How do Ra, Rz, and N convert to each other?
Ra 
N
N1-N12 values directly represent specific Ra values
Ra
Rz
Ra and Rz are not directly convertible because they represent two different things.
However:
- A range of all possible values based on statistics can be specified.
- For each given Ra value there is a range of Rz values and vice versa. The higher the number (rougher the surface) the less accurate is the range. You can see it on the following chart: A part with roughness 3.2 µm in Ra can have Rz roughness from 11.5 to 34.7 µm while 50 µm Ra can represent from 156.2 to 272.6 µm
Rz
N
Since N is a representation of Ra it is not possible to convert directly between Rz and N:
- Each Rz value is first converted to Ra range
- And then the corresponding classes of N are found for both numbers of Ra.
What is the standard surface finish in CNC machining? What options does Xometry offer?
3.2 μm Ra is usually the standard surface finishing for machined parts. It is suitable for most consumer parts and sufficiently smooth, but it contains visible cut marks. It is the default surface roughness applied at Xometry unless otherwise specified. Smoother surfaces that are also usually achievable with CNC machining and offered by Xometry are 1.6 μm Ra, 0.8 μm Ra and 0.4 μm Ra.
| Surface roughness | Cut marks | Can be used for | |
|---|---|---|---|
Rougher Cheaper
Smoother More expensive
|
|||
| 3.2 μm Ra | Visible |
|
|
| 1.6 μm Ra | Slightly visible |
|
|
| 0.8 μm Ra | Not visible |
|
|
| 0.4 μm Ra | Not visible |
|
