Surface Roughness Chart: Ra µm to µin & Parameters

Published September 22, 2026 · MfgWorkbench editorial reference

This chart converts the unit of a roughness value; it does not convert one roughness parameter into another. Keep the parameter name attached to every number.

Ra, Rq and Rz measure different things

Ra is the arithmetic mean of absolute profile deviations over the defined evaluation. Rq uses a root-mean-square calculation. Rz is a height-based parameter whose definition depends on the specified standard and evaluation rules. Two profiles can have the same Ra but different peak or valley behavior.

There is no universal exact multiplier that converts Ra to Rz. A process-specific empirical relationship might help planning only when supported by measurements and the applicable definitions. It must not replace the parameter required by a drawing.

An exact unit conversion example

One microinch is 0.0254 micrometers. Therefore 1.6 µm Ra = 62.992126 µin Ra. A drawing calling for 63 µin Ra is close to 1.6 µm after rounding, but the acceptance method still follows the drawing. The unit conversion says nothing about waviness, form or the appearance of directional tool marks.

Likewise, a calculated turning cusp height is not measured Ra. The turning-finish calculator estimates ideal geometry; the surface can contain vibration marks, tearing and other deviations absent from that model.

Make the inspection reproducible

Check the drawing’s standard edition, parameter, filter or cutoff requirements, evaluation length and measurement direction. A short trace can miss longer-wavelength features, and measuring along a lay can differ from measuring across it. Record the instrument setup with the result.

Avoid labeling a manufacturing process with a guaranteed roughness number. Turning, milling, grinding and printing each span broad outcomes depending on tooling, material and setup. Choose a process and verify it on representative parts rather than treating a generic process chart as proof of capability.