Surface Finish Reference

Ra, Rz, RMS, and machining process finish ranges for drawings and quoting.

Converter

Ra (micrometer)

Rz (est. ≈4×Ra)

6.40µm

RMS (est. ≈1.11×Ra)

1.78µm

Ra (µin)

63µin

N grade

N7ISO

Rule of thumb: Rz is often estimated as 4× Ra, but actual ratios vary with process and measurement standard.

Lapping / polishing0.025-0.2 µm Ra

Seals, optics, bearing surfaces

Grinding0.2-1.6 µm Ra

Precision shafts, datums, close fits

Reaming / honing0.4-1.6 µm Ra

Accurate bores and bushings

Fine turning0.8-3.2 µm Ra

Good cosmetic and functional finish

Fine milling1.6-3.2 µm Ra

Typical CNC machined faces

General milling3.2-6.3 µm Ra

Standard non-cosmetic machining

EDM sinker1.6-6.3 µm Ra

Texture depends on power setting

Wire EDM0.8-3.2 µm Ra

Skim cuts improve finish

Saw cut / roughing12.5-50 µm Ra

Prep stock, hidden surfaces

Ra µm	Ra µin	Rz µm approx.	RMS µm approx.	Typical callout
0.025	1	0.10	0.03	Precision
0.05	2	0.20	0.06	Precision
0.1	4	0.40	0.11	Precision
0.2	8	0.80	0.22	Precision
0.4	16	1.60	0.44	Precision
0.8	31	3.20	0.89	Machined
1.6	63	6.40	1.78	Machined
3.2	126	12.80	3.55	Machined
6.3	248	25.20	6.99	Rough
12.5	492	50.00	13.88	Rough
25	984	100.00	27.75	Rough
50	1968	200.00	55.50	Rough

Converts surface roughness between Ra, Rz, RMS, and ISO N grades and relates them to typical machining processes.

Inputs: A roughness value
Its parameter and unit (µin / µm)
Assumptions: Rz ≈ 4–7× Ra depending on process; the tool uses a typical ratio
RMS ≈ 1.11 × Ra for a sinusoidal profile
Process ranges are typical, not guaranteed
Sources: ASME B46.1
ISO 1302 / ISO 21920 roughness grades
Valid range: Ra 0.025–50 µm (1–2000 µin).
Limitations: Ra-to-Rz conversion is an approximation and varies by process
No waviness, lay, or cutoff-length handling
Drawing callouts may specify sampling length and filter — follow the drawing

Values are starting points for planning — verify against your tooling and machine documentation before cutting. Report an issue with this tool →