Technical Reference
Ra and Rz Surface Roughness: Reading Measurement Results
Interpret Ra and Rz with their units, standards and measurement conditions, without assuming a universal conversion.
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Ra and Rz describe different aspects of a measured surface profile. Ra expresses an arithmetic average of absolute profile deviations; Rz is a height-related parameter whose exact definition must be read in the specified standard. Neither value is complete without units and measurement conditions. A drawing that states only “surface finish” leaves those matters unresolved. KEYENCE parameter explanation
Profile roughness and parameter names
This page concerns a measured profile rather than a complete three-dimensional surface. Ra summarises the absolute height deviations of that profile from its mean line. Rz focuses on profile-height information and needs its standard-specific definition. The same unit does not make them interchangeable. Profile parameter explanation
A requirement and a reported result also play different roles: a requirement states the permitted condition, while a result describes a measurement made using stated settings. Retain both when comparing the two.
What to read beside the number
| Item | Why it belongs in the record |
|---|---|
| Parameter name | Ra and Rz are not interchangeable labels |
| Unit | A bare decimal cannot identify the physical magnitude |
| Standard and edition | Parameter definitions and evaluation rules depend on the chosen system |
| Filter and evaluation settings | Results need a stated measurement basis |
| Location and measurement direction | The record must identify the surface actually assessed |
This table is an editorial reporting checklist, not a replacement for the governing surface-texture standard or an instrument procedure.
An average is not the complete surface
KEYENCE explains that different profiles can share the same Ra while having different peaks, valleys or local defects. That is why a result should be interpreted against the functional requirement rather than treated as a general quality score. Ra interpretation and limitations
For example, two hypothetical records both showing Ra = 1.6 μm do not establish that the surfaces have the same texture or performance. A separate Rz result cannot be calculated from that number alone here. This is a reading example, not a suggested finish for a machined part.
Keep the standard transition visible
Mahr describes calibration certificates supporting both earlier standards and the ISO 21920 system during the transition. Its explanation identifies changes to terminology and evaluation settings. A result from one system should not silently be relabelled as a result from another. Mahr transition explanation
Use the definition and evaluation rules of the specified edition when interpreting Rz. No fixed Ra-to-Rz ratio or default evaluation length is prescribed here.
Limits and connected reading
Agree the parameter, edition, conditions and acceptance requirement with the responsible designer and measurement specialist. This page does not certify compliance or choose a universal acceptance rule. For the distinction between allowed variation and demonstrated capability, see CNC dimensional tolerances.
Edited and reviewed by: 2kp.cc Editorial Team
Scope & conditions: Profile-roughness reporting concepts; excludes a complete standard algorithm, acceptance limits and automatic Ra-to-Rz conversion.
Commercial disclosure: This article is not sponsored and contains no paid recommendations.
Sources
- How Surface Height Is Quantified — From Ra to Rz — Provided by: KEYENCE · Accessed: 2026-10-10
- DAkkS-accredited calibration according to new roughness standard — Provided by: Mahr · Accessed: 2026-10-10
Important revisions
- — Clarified the profile-based scope of Ra and Rz and kept standard-specific evaluation rules explicit.