Surface Roughness Converter

Formula

• Rz ≈ 6 × Ra
• Rq ≈ 1.11 × Ra
• Rt ≈ 7.5 × Ra
• Ra (µin) = Ra (µm) × 39.37

About this calculator

Part of our GD&T & Metrology Calculators collection. True position, fits, ISO tolerances, and surface roughness.

How it works

Enter one roughness parameter and the converter estimates the related Ra, Rz, Rq, and Rt values using standard approximation ratios. Micro-inch and metric Ra convert directly. Closest N grade maps your value to the ISO 1302 scale.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

Input Ra = 1.6 µm (typical ground surface).

1. Rz = 6 × 1.6 = 9.6 µm
2. Rq = 1.11 × 1.6 = 1.78 µm
3. Closest ISO N grade = N7

Result: Ra 1.6 µm ≈ 63 µin, N7 finish.

When to use

• Converting between Ra, Rz, Rq, Rt, and micro-inch
• Mapping measured Ra to ISO N-grade for drawings
• Comparing supplier finish reports in different units

Limitations

• Rz/Rq/Rt ratios are approximations; actual profile shape varies by process
• N-grade mapping uses closest log match, not exact standard band
• Does not compute from raw profile data

FAQ

Ra vs Rz?

Ra is average roughness. Rz is average peak-to-valley height over sampling lengths — typically about 4 to 7× Ra depending on process.

What is N7?

ISO 1302 N7 corresponds to Ra ≈ 1.6 µm — common for general machined surfaces.