Surface roughness calculator: Ra ↔ Rz conversion, roughness classes and drawing symbol

Enter Ra or Rz — the calculator converts it, finds the roughness class and the preferred value, suggests machining processes and draws the roughness symbol.

Roughness parameters

Ra — arithmetic mean deviation, Rz — ten-point height, measured over the sampling length. Preferred Ra: 100 … 0.012 µm.

Ra to Rz

No exact formula; Rz ≈ 4·Ra for rough and ≈ 5·Ra for fine surfaces, per class table.

Drawing symbol

Two lines at 60°; a bar for material removal, a circle for no removal; value under the bar, process above it, lay symbol at the right.

FAQ

How to convert Rz to Ra?

Approximately Ra ≈ Rz/4 for rough and Rz/5 for fine surfaces.

What does Ra 3.2 mean?

Mean deviation not above 3.2 µm — finish turning or milling, about Rz 12.5–16.

What do the symbol types mean?

Basic — any process; with bar — machining; with circle — no material removal.

Sampling length for Ra 1.6?

0.8 mm.

What roughness does turning give?

Rough Ra 12.5–50, finish 1.6–6.3, fine 0.4–1.6 µm.