Surface roughness / Ra guide
Ra surface roughness: what does Ra 1.6 µm mean?
Ra describes the arithmetic mean of the absolute roughness-profile heights relative to the mean line. A Ra 1.6 µm marking states that parameter and value; the drawing supplies its role and measurement requirements.
Average height variation
Read its role in the drawing
Micrometres
Illustrative marking only. This is not a complete surface-texture symbol or a pass/fail rule.
What is Ra? Arithmetic mean roughness
In the legacy parameter definitions explained by Mitutoyo (JIS B 0601:2013 / ISO 4287:1997, Amd.1:2009), Ra is the average absolute height of the roughness profile relative to its mean line. It summarizes average height variation rather than the largest individual peak or valley.
Definition and edition: Mitutoyo parameter definitions ↗.
Ra units: 1.6 µm is about 63 µin
Using the exact relation 1 µin = 0.0254 µm, Ra 1.6 µm ÷ 0.0254 = approximately Ra 62.992126 µin, or 63 µin rounded. This changes the unit, not the roughness parameter.
Unit basis: NIST international inch definition ↗.
Convert another Ra value →Ra alone does not determine Rz
Rz describes peak-to-valley characteristics under the specified definition. An average does not uniquely determine the extrema, so a universal multiplier cannot supply an equivalent Rz value.
For a simple mathematical illustration, the samples [−1, −1, 1, 1] and [−2, 0, 0, 2] both have mean absolute height 1, but peak-to-valley ranges of 2 and 4. These are teaching samples, not standard-compliant roughness measurements.
Older standards also used Rz for ten-point roughness. Keep the parameter definition and standard edition with the measurement.
Convert units without changing the parameter →What to record with the number
A roughness value needs its measurement context. Take these fields from the drawing and the applicable inspection procedure.
- Parameter, value, unit and whether the drawing specifies a limit.
- Standard and edition, including any drawing-specific instructions.
- Profile/filter settings and evaluation length.
- Measurement direction, instrument/stylus setup and measurement location.
“Ra 1.6 µm” alone does not establish filter settings, evaluation length or an acceptance decision. Ask for the missing inspection requirements before comparing measured results.
Measurement context: Mitutoyo, Profiles and filters, PDF pp. 2–3 ↗ (guide references older DIN/ISO editions).
Sources and scope
Reviewed 23 October 2026. These links support specific definitions, units and measurement context.
Definitions and version differences — Mitutoyo
Surface roughness fundamentals ↗: parameter-definition sections explicitly reference JIS B 0601:2013 / ISO 4287:1997, Amd.1:2009. Older uses of Rz include ten-point roughness; record the edition before comparing values. This explanation is not a reproduction of the ISO 21920 definitions.
Current profile-texture series — ISO public catalogue
- ISO 21920-1:2021 ↗: graphical indication.
- ISO 21920-2:2021 ↗: terminology, definitions and parameters; corrected English version 2022-06.
- ISO 21920-3:2021 ↗: specification operators.
Public scope and lifecycle records were checked. The full standards were not reviewed here. Use the edition required by the drawing; this page does not select default filters or inspection rules.
Exact unit basis — NIST
SI Units — Length ↗, international inch definition: 1 inch = 25.4 mm exactly. Therefore 1 µin = 0.0254 µm exactly.