Surface Finish Converter

An Ra of 1.6 µm (a common finish-machined surface, N7) converts to 62.99 µin Ra, 1.776 µm RMS, and 6.4 µm Rz.

The formulas

Ra vs. Rz vs. RMS

Ra (arithmetic average roughness) is the most widely specified parameter — it averages the absolute deviations of the surface profile from its mean line over the evaluation length. It is simple and repeatable, but it can hide isolated deep scratches or peaks.

Rz (mean peak-to-valley height) averages the five highest peaks and five deepest valleys in the sampling length. It is more sensitive to occasional defects than Ra and is commonly called out on European drawings alongside or instead of Ra.

RMS (Rq, root mean square roughness) weights larger deviations more heavily than Ra. For a Gaussian surface profile, RMS ≈ 1.11×Ra. The US historically used RMS on older drawings; modern practice favours Ra per ISO 4287.

ISO surface-roughness N-grades

ISO 1302 assigns N-grades (N1–N12) to standard Ra values. Drawings may specify the N-grade, the Ra value, or both. The triangle symbol (△) on a drawing indicates a machined surface; additional markings specify the direction of lay and maximum waviness.

Typical process finishes

Each machining process produces a characteristic roughness range. The values below are typical for well-maintained tooling; actual results depend on speed, feed, tool geometry, and workpiece material.