Use the following formulas to convert average roughness (Ra) to other commonly used surface roughness parameters:
Rt = Ra x 8.7
Rz = Ra x 7.2
Rz ISO = Ra x 7.6
Rmax = Ra x 8.0
Rp = Ra x 3.6
RPM = Ra x 2.9
RMS = Ra x 1.1
The conversion formulas work regardless of whether Ra is given in microinches (µin) or micrometers (µm).
To convert metric Ra to centerline average (CLA), use the following formula:
CLA (µin) = Ra (µm) x 40