You can convert different roughness values to the more commonly used Ra roughness value using the table below. This table was created to provide an indication. It gives approximate results.
Conversion Formulas
Conversion Formula: F = Ra x Coefficient
Reverse Conversion: Ra = F / Coefficient
Coefficient Table
| Roughness Value (F) | Coefficient | Description |
|---|---|---|
| Rt | 8.7 | Total profile height |
| Rz | 7.2 | Average maximum profile height (DIN) |
| Rz ISO | 7.6 | Average maximum profile height (ISO) |
| Rmax | 8.0 | Maximum roughness depth |
| Rp | 3.6 | Average peak height |
| RPM | 2.9 | Maximum peak height |
| RMS (Rq) | 1.1 | Root mean square roughness |
Calculation Examples
Example 1: Ra to Rz Conversion
If Ra = 0.8 µm, what is Rz?
Rz = Ra x 7.2 = 0.8 x 7.2 = 5.76 µm
Example 2: Rz to Ra Conversion
If Rz = 1 µm, what is Ra?
Ra = Rz / 7.2 = 1 / 7.2 = 0.14 µm
Example 3: Ra to RMS Conversion
If Ra = 1.6 µm, what is RMS (Rq)?
RMS = Ra x 1.1 = 1.6 x 1.1 = 1.76 µm
Example 4: Rt to Ra Conversion
If Rt = 17.4 µm, what is Ra?
Ra = Rt / 8.7 = 17.4 / 8.7 = 2.0 µm
Warning: These coefficients are approximate values. Actual conversion ratios may vary depending on material, machining method and surface profile. Direct measurement is recommended for precision applications.
Related Articles
- Metric Surface Roughness Chart — Symbols, Ra values and roughness by manufacturing method
- Tolerances by Machining Method — IT tolerance values chart
Prepared by: Yamac Aksan