Surface Roughness Converter (Ra / Rz / Rmax)
Convert between Ra, Rz, Rmax and RMS surface roughness parameters and map them to ISO 1302 grade numbers, ASME B46.1 values and traditional N grades. Conversion ratios depend on the manufacturing process, so results are flagged with the applicable ratio band.
How to use this calculator
- Enter a known roughness value and select its parameter type (Ra, Rz, Rmax or RMS in micrometres or microinches).
- Select the manufacturing process to apply the correct Ra-to-Rz ratio band.
- Read the equivalent values, the ISO grade number and the recommended callout string.
Frequently asked questions
Is there an exact formula to convert Ra to Rz?
No. Ra and Rz describe different features of the same profile, so the ratio varies with the shape of the profile. A commonly used approximation for ground or turned surfaces is Rz about 4 to 7 times Ra, but a honed plateau surface can be much lower. Treat conversions as estimates, not specifications.
What is the difference between ISO and ASME roughness grades?
ISO 1302 uses grade numbers N1 to N12 tied to Ra limits in micrometres. ASME B46.1 historically used RMS microinch values. Both appear on drawings, so this calculator reports both.
How do microinches relate to micrometres?
1 micrometre (um) equals 39.37 microinches (uin). Ra 0.8 um is Ra 32 uin.