Use the following formulas to convert average roughness (Ra) to other commonly used surface roughness parameters:

R_t = R_a x 8.7

R_z = R_a x 7.2

R_z ISO = R_a x 7.6

R_max = R_a x 8.0

R_p = R_a x 3.6

RPM = R_a x 2.9

RMS = R_a x 1.1

The conversion formulas work regardless of whether R_a is given in microinches (µin) or micrometers (µm).

To convert metric R_a to centerline average (CLA), use the following formula:

CLA (µin) = R_a (µm) x 40