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Surface Roughness Conversions

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