Reference · Released

Surface Finish Conversion

Convert between common surface roughness units: Ra (µm and µin), Rz (µm), RMS, and ISO grade N. Enter any one value and the tool shows the rest. Helps when reading drawings that mix metric and imperial finish specifications.

Ra — micrometers (µm)

Ra — microinches (µin)

Rz (Rt rough avg) — µm

RMS — µm (approx.)

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Rz (approx.)

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RMS (approx.)

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ISO grade / typical process

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Conversion relationships

1 µin = 0.0254 µm, so Ra(µin) = Ra(µm) × 39.37
Rz ≈ 4 × Ra (empirical, for general machining; varies 3–5× by process)
RMS ≈ 1.11 × Ra (for Gaussian surfaces)

ISO surface roughness grades (N series):

Grade	Ra µm	Ra µin	Typical process
N1	0.025	1	Lapping, honing
N2	0.05	2	Superfinishing
N3	0.10	4	Grinding
N4	0.20	8	Fine grinding
N5	0.40	16	Finish turning / boring
N6	0.80	32	Finish milling
N7	1.60	63	General machining
N8	3.20	125	Rough milling / turning
N9	6.30	250	Roughing
N10	12.5	500	Heavy roughing