# Surface Finish (Ra) Ra is the arithmetic average roughness of a surface, stated in microinches or micrometres. A capability page must state process, materials, tolerances, capacity, certifications, and industries served, and surface finish is part of that. # Surface Finish (Ra)
Ra is the arithmetic average roughness of a surface, stated in microinches or micrometres. A capability page must state process, materials, tolerances, capacity, certifications, and industries served, and surface finish is part of that.
**Ra averages the vertical deviations of a surface profile from its mean line.** A finish of 32 microinch Ra is smoother than 125 microinch Ra, and the value is measured with a profilometer traversing the surface rather than estimated by eye. The two unit systems are both common and are not interchangeable in casual reading. 32 microinch is roughly 0.8 micrometre. A drawing calling one while a supplier quotes the other is a routine source of a mismatch found at inspection. **Ra does not describe everything about a surface.** Two surfaces with the same Ra can behave differently in sealing, wear, or coating adhesion, because Ra is an average and says nothing about the shape or spacing of the peaks. Where the function depends on that, a drawing calls additional parameters. **Finer finishes usually require a secondary operation.** Grinding, honing, lapping, polishing, or a specific insert and feed strategy each add time and cost, which is why a finish specified tighter than the function needs is the same kind of avoidable cost as an over-tight tolerance. For search, surface finish is one of the specifications most often left off a capability page and one a constrained supplier prompt filters on directly. A page stating an achievable range by process and material is matchable; a page describing "excellent surface quality" is not. The same rule that governs tolerance governs this: a buyer needs a value they can compare against a drawing, and a claim without a number cannot be compared. See [capability page structure](/content/capability-page-structure) for where the value belongs, and [tolerance](/glossary/tolerance) for the specification it is usually stated alongside. --- Source: https://manufacturingseo.ai/glossary/surface-finish-ra/ Last reviewed: 2026-08-08 Author: ManufacturingSEO.ai