Surface Finish Converter

Convert surface roughness Ra between micrometers and microinches, find the nearest ISO grade number (N1–N12), and estimate RMS and Rz. Calculates in your browser.

ISO roughness grade

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About surface finish

Ra (the arithmetic average roughness) is the most common surface-finish measure. This tool converts Ra between micrometers and microinches, maps it to the nearest ISO grade number, and estimates the related RMS (Rq) and Rz parameters using common rule-of-thumb factors:

RMS ≈ 1.11 × Ra  ·  Rz ≈ 4 × Ra

RMS and Rz factors vary with the surface profile, so treat them as approximations. The Ra↔grade and µm↔µin conversions are exact.

What this does

This converter changes surface roughness Ra between micrometers and microinches, finds the nearest ISO grade number (N1–N12), and estimates the matching RMS and Rz values.

How to use it

  1. Enter an Ra value in µm or µin.
  2. Read the converted Ra and ISO grade.
  3. Check the estimated RMS and Rz.

How it works

Ra is the arithmetic average roughness, the mean deviation of the profile from its centerline. The µm↔µin and grade conversions are exact; RMS (≈ 1.11 × Ra) and Rz (≈ 4 × Ra) use common rule-of-thumb factors.

µin = µm × 39.37

Example

An Ra of 1.6 µm is about 63 µin, which is ISO grade N7.

Sources & methodology

Last updated .

Frequently asked questions

What is Ra?

Ra is the arithmetic average roughness, the mean deviation of the surface profile from its centerline. It is the most widely quoted surface-finish parameter.

How accurate are the RMS and Rz values?

They use common rule-of-thumb factors (RMS ≈ 1.11 × Ra, Rz ≈ 4 × Ra). Actual ratios depend on the surface profile, so treat them as estimates. The µm↔µin and grade conversions are exact.

Is my data stored?

No. Everything is calculated locally in your browser.