Tolerances & finishes
Read the requirement before comparing the result.
Start with the parameter on the drawing. Keep the measurement conditions with the value.
Ra 1.6 µm
Separate the parameter, number and unit.
Read the marking →02 / CONVERTµm or µin?
Convert units for the same parameter.
Open the converter →03 / COMPARERa and Rz
Understand why one does not determine the other.
Compare the meanings →04 / CHECKMeasurement context
Find the conditions needed alongside the number.
See the checklist ↓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).
Read the rest of the requirement
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.