Surface Finish Standards

Ra (Roughness Average)

Ra is the arithmetic average of absolute profile height deviations. Units: µm or µin. Typical values: CNC 0.4-6.3 µm, grinding 0.1-1.6 µm, polishing 0.05-0.4 µm. Most widely used parameter.

Rz (Average Max Height)

Rz averages the 5 highest peaks minus 5 deepest valleys. Rz is typically 4-6× Ra. Conversion: Rz ≈ 4.0 × Ra. Use for sealing surfaces and dynamic contact.

RMS (Root Mean Square)

RMS ≈ 1.11 × Ra. ASME B46.1: RMS is older standard, superseded by Rq. Common in US aerospace and medical drawings.

Process Capability