Surface roughness Ra vs Rz
Read surface roughness requirements, distinguish Ra from Rz, convert units correctly and specify the measurement conditions for a functional machined surface.
Ra and Rz describe different aspects of a profile
Ra averages absolute height deviations of an evaluated roughness profile. Rz is a height parameter whose definition and evaluation depend on the stated standard and edition.
Frequent small deviations
Eight illustrative height samples alternate between −1 and +1 unit. Their mean absolute height is 1 unit; the sample peak-to-valley span is 2 units.
A few large deviations
Six samples are zero, with one at +4 and one at −4 units. The mean absolute height is still 1 unit; the sample peak-to-valley span is 8 units.
A mathematical illustration in arbitrary units. These are not instrument traces, Ra/Rz test results or a conversion chart; real evaluation requires the specified filtering and measurement method.
| Parameter | What it emphasizes | Do not infer |
|---|---|---|
| Ra | Average absolute profile-height deviation under the specified evaluation. | That isolated peaks, deep valleys, scratches or directional texture are acceptable. |
| Rq | Root-mean-square profile height; larger deviations carry more weight. | That Rq and Ra are interchangeable values. |
| Rz | A profile-height measure evaluated under the named standard. | That an old JIS, ISO or other Rz definition is automatically the same as another. |
| Rt | Total height over the applicable evaluation length. | That it is the same parameter or evaluation as Rz. |
A parameter name and value are only part of a specification. Filtering, evaluation length, trace direction and acceptance rules affect the result.
Convert units, not one roughness parameter into another
Micrometers and microinches are length units. Ra and Rz are different calculations. A valid unit conversion does not make a fixed Ra-to-Rz multiplier valid.
For example, Ra 1.6 µm converts to approximately Ra 63 µin. It does not convert to Rz 6.3 µm by a universal rule. The relationship between average and peak-related parameters changes with the actual profile.
| Micrometers (µm) | Microinches (µin) |
|---|---|
| 0.4 | 15.748 |
| 0.8 | 31.496 |
| 1.6 | 62.992 |
| 3.2 | 125.984 |
| 6.3 | 248.031 |
1 µin = 0.0254 µm exactly. Displayed conversions are rounded. This table does not specify a preferred roughness, an N-grade equivalence or a machining guarantee.
Choose texture requirements for the contact
| Surface | Engineering question | What to define |
|---|---|---|
| Seal contact | Can the texture or a scratch create a leakage path? | Required parameters, lay direction, defects and the final surface condition specified by the seal design. |
| Sliding or lubricated contact | How should the surface retain lubricant and support contact? | Relevant profile parameters, direction and the mating material; lower Ra alone is not a complete tribology design. |
| Press-fit seat | Will surface peaks settle during assembly or affect contact? | Size, form and texture requirements together, in the final condition. |
| Cosmetic face | What appearance will the customer accept? | A representative visual reference and viewing conditions; Ra alone does not define colour, gloss or tool-mark appearance. |
| Coating or adhesive interface | What preparation does the process need? | Cleanliness and preparation required by the coating or adhesive system rather than a universal roughness target. |
Make the measurement repeatable
Two reports can disagree because they measured differently. Agree the evaluation conditions before a supplier accepts a texture requirement.
- Name the standard and edition. Use the drawing’s governing system. ISO 21920 addresses profile surface texture; legacy ISO 4287/4288 drawings require their own interpretation rather than a silent conversion.
- Identify the actual surface. Mark the face and measurement region. Edge transitions, local damage and inaccessible recesses can make an unspecified trace ambiguous.
- Define the evaluation setup. Use the applicable filtering, evaluation length, trace direction and instrument settings. Confirm that the probe or optical method can access the feature.
- Measure the delivered condition. State whether the requirement applies after machining, grinding, blasting, plating or another final operation.
Give the surface a complete callout
| Field | What to communicate |
|---|---|
| Parameter and limit | Ra, Rz or another required parameter, units and the acceptance limit under the governing standard. |
| Location | The functional face or controlled zone, not an unclear note applied to every surface. |
| Lay or process constraint | Direction or process only where it controls function; avoid prescribing an unnecessary operation. |
| Evaluation | Applicable measurement conditions and any non-default acceptance requirements. |
| Final state | Before or after coating/finishing, plus protected or masked surfaces. |
This checklist is an RFQ preparation aid, not a substitute for the complete drawing-symbol rules in the applicable standard.
Quote the surface together with the geometry
A surface inside a deep pocket, on a long slender shaft or beside an interrupted feature may require a different route from an accessible flat face. Material, tool access, vibration, finishing and measurement access affect feasibility.
Ask the supplier to identify the operation and inspection that will achieve the requirement. Apply demanding texture controls to the faces that need them; a blanket “polish all surfaces” note can add work without protecting the actual function.
Continue to CNC machining design, surface-finish selection or inspection requirements.
Frequently asked questions
Can I convert Ra to Rz with a multiplier?
There is no universal exact conversion. Ra and Rz emphasize different profile features, and the relationship depends on the surface and evaluation method. Specify and measure the parameter required by the drawing.
What is Ra 1.6 in microinches?
Ra 1.6 micrometers is approximately Ra 63 microinches because 1 microinch equals 0.0254 micrometers. This changes the unit, not the roughness parameter.
Is lower Ra always better?
No. The required texture depends on sealing, sliding, lubrication, retention, bonding or appearance. A lower average alone does not establish that peaks, valleys, lay or surface defects meet the application.
Does a surface roughness value control flatness?
No. Surface texture and overall geometric form are different requirements. A surface can have a low roughness value while still being bowed, tapered or out of flatness.
Should roughness be checked before or after coating?
State the condition that matters to the function. Coating, blasting, polishing or grinding can change the delivered texture; the drawing and inspection plan should identify the final operation before measurement.
Sources and further reading
- NIST — surface roughness and step-height calibrationsParameter distinctions and measurement-condition/uncertainty context.
- ISO 21920-1:2021 — indication of surface texturePublic scope for profile-method indications in technical documentation; use the contractually applicable edition.
- ISO 21920-2:2021 — terms and parametersTerms, definitions and parameters for surface texture by profile methods.
- Mitutoyo — surface roughness measurement resourcesMeasurement guidance; older published guides use legacy standard editions.
Specify the surface that makes the part work.
Upload your CAD and drawing with the material, quantity, finishing and critical dimensions. Confirm the manufacturing route before production.