Ra vs Rz Surface Roughness: Definitions and Conversion

Ra is the arithmetic mean of the absolute height deviations of a surface profile from its mean line. Rz is the height of the highest peak plus the depth of the deepest valley within a measuring section, averaged over the sections. No single factor converts one into the other, because the ratio depends on the shape of the profile as well as its height.

Both parameters are defined in ISO 21920-2:2021, which replaced ISO 4287 in December 2021. Drawings still cite the old documents, and the old and new rules differ in how they average and in the default acceptance rule, so the edition matters.

What are Ra and Rz?

Both are computed from the roughness profile, the trace of a stylus or optical probe across the surface after form and waviness have been filtered out. Ra reports the average height of that trace. Rz reports its extremes. A deep scratch raises Rz clearly and Ra only slightly, because Ra averages the scratch over the whole length.

Table 1. Ra, Rz and Rzx in ISO 4287 and ISO 21920-2

Source: ISO 4288:1996, clause 4.1.2 (five sampling lengths, average parameter estimate); changes after the Digital Surf Surface Metrology Guide on ISO 21920-2. The standards texts are paywalled.

The wording changed too. "Sampling length" became "section length", and "number of sampling lengths" became "number of sections". The default stays at five sections per evaluation length. Digital Surf notes that most ISO 4287 parameters were carried over with slightly different definitions, so the same surface can give a slightly different value under the new standard.

Is ISO 4287 still valid?

No. ISO published the three parts of ISO 21920 on 2021-12-20. Part 2 replaced ISO 4287:1997 and the two ISO 13565 parts on stratified surfaces. Part 1 replaced ISO 1302:2002, the standard for surface texture symbols on drawings. Part 3 replaced ISO 4288:1996, the rules for measuring settings and comparing results with limits. DIN adopted parts 2 and 3 as DIN EN ISO 21920-2 and -3 in December 2022, replacing DIN EN ISO 4287 and DIN EN ISO 4288.

Status as of 2026-10-30: all three parts are published and ISO has marked them "to be revised" since 2025-10-07. ISO lists committee drafts of new editions as under development, so the 2021 editions remain the current text.

Polytec, a metrology vendor, writes that drawings prepared under the older standards do not lose their validity. The practical consequence is that a drawing citing ISO 1302 or ISO 4288 without a year points to a withdrawn document. Write the edition on the drawing.

Can you convert Ra to Rz?

Not with one factor. The standards define each parameter on its own and give no conversion between them. The ratio follows from the profile shape, which you can check for ideal profiles directly from the definitions.

Table 2. Ra and Rz for ideal profiles (A = amplitude about the mean line, h = peak-to-valley height)

Source: calculated from the Ra and Rz definitions in ISO 4287 and ISO 21920-2; the arc case takes each feed mark as a parabola.

Real surfaces are not ideal profiles. A ground face with a few pits and a milled face with a regular feed pattern can share one Ra and differ widely in Rz. A conversion table or a "multiply by 4" rule can only describe an average over many parts of one process. It cannot serve as an acceptance criterion. If the drawing needs Rz, ask for Rz, and if a customer states only Ra, do not infer an Rz limit from it.

What roughness does turning produce?

The ISO 21920 series defines parameters and measuring rules. It contains no table of Ra by manufacturing process. DIN 4766-1:1981-03 once listed attainable Rz values by process and is withdrawn. We therefore do not quote process ranges from handbooks or web charts.

For one process the geometry can be calculated. A turning tool with nose radius r that advances f per revolution leaves arcs of height Rz = f² / (8r), and Ra ≈ 0.032 × f² / r. This holds while the cut engages only the nose radius.

Table 3. Ideal turned profile, nose radius r = 0.8 mm, µm

Source: calculated from the tool-nose geometry, Rz = f² / (8r); the Ra factor 0.032 follows from integrating the parabolic profile.

These values describe an ideal profile. Measured values on real parts include tool wear, vibration and material effects on top of the geometry. The formula still shows how the feed acts: halving it cuts both Rz and Ra to a quarter, so on one insert the step from Ra 1.6 µm to Ra 0.4 µm is a feed change from 0.2 to 0.1 mm/rev. Whether a given setup reaches the value is a matter for a measurement on the first article. See CNC turning and CNC milling for the processes.

Which measuring settings and acceptance rule apply?

Two surfaces measured with different filters give different Ra values. ISO 21920-3 fixes this with five setting classes, Sc1 to Sc5. For Ra and Rz the class comes from the specified limit.

Table 4. Default settings by roughness limit, ISO 21920-3

Source: ISO 21920-3:2021 setting classes, as reproduced in the Polytec guideline "Roughness evaluation according to ISO 21920". Five sections per evaluation length.

A drawing with Ra 1.6 µm lands in Sc3 and is evaluated over 4 mm. Ra 3.2 µm lands in Sc4 and needs 12.5 mm of measurable surface. A short land or a narrow groove may not offer that length, and the drawing then has to name the settings it wants.

The acceptance rule changed as well. ISO 4288:1996, clause 5.2, applied the 16 % rule to a limit written without the index max: a surface passes if no more than 16 % of the measured values exceed the upper limit. A limit written with max, such as Rz1max, followed the max rule, where no value may exceed it. ISO 21920-1 reverses the default. CMM Quarterly and Polytec both report that the maximum rule applies unless the drawing says otherwise, and that the 16 % rule must be requested explicitly. The same number on an old and a new drawing can therefore be a different test.

How do you specify surface roughness on a drawing?

Use the ISO 21920-1 symbol and state, for each functional surface, the parameter, the limit, the acceptance rule and, where the default does not fit, the setting class or filter. A short checklist before a drawing goes out for quotation:

• The parameter is named: Ra for average texture, Rz where one peak or scratch matters.
• The limit is a value a measurement can verify, in µm, and its upper or lower role is clear.
• The rule is stated, or the drawing names the standard edition so the default is known.
• The surface is identified, so that a decorative face does not carry a sealing-face limit.
• Features too small for the Sc evaluation length carry explicit settings.

Roughness limits belong with the other title block entries in the technical drawing checklist. They sit beside, not inside, the general tolerances of ISO 2768, which do not cover surface texture, and the CNC machining tolerances handbook shows how both combine with fits on one part. Coatings and plating change the final roughness again, see surface treatments for machined parts.

Frequently asked questions

What is the difference between Ra and Rz?

Ra is the arithmetic mean of the absolute height deviations of the roughness profile from its mean line. Rz is the height of the highest peak plus the depth of the deepest valley within a section, averaged over the sections. A single scratch changes Rz noticeably and Ra only slightly.

Is there a formula to convert Ra to Rz?

No. Neither ISO 4287 nor ISO 21920-2 relates the two parameters. For an ideal square wave Rz is 2 × Ra, for a sine wave about 3.14 × Ra, for a triangular wave 4 × Ra. Measure both if a drawing needs both.

Is ISO 4287 still valid?

No. ISO 21920-2:2021 replaced ISO 4287:1997, ISO 21920-1:2021 replaced ISO 1302:2002 and ISO 21920-3:2021 replaced ISO 4288:1996. As of 2026-10-30 ISO lists all three parts as published and marked for revision.

Does Ra 1.6 on a drawing mean the 16 % rule or the max rule?

Under ISO 4288:1996, a limit without the index max meant the 16 % rule. Under ISO 21920-1 the maximum rule is the default, and the 16 % rule has to be written on the drawing. For drawings that do not name an edition, ask which one applies.

Should a drawing specify Ra or Rz?

Specify the parameter that matches the function and state the acceptance rule. Ra describes the average texture. Rz reacts to isolated peaks and scratches, so it suits surfaces where one peak matters, such as a sealing face.

Sources

1. ISO 21920-2:2021, Surface texture: Profile, Part 2 (ISO catalogue, stage 90.92 dated 2025-10-07)
2. ISO 21920-1:2021, Surface texture: Profile, Part 1, Indication of surface texture (ISO catalogue)
3. ISO 21920-3:2021, Surface texture: Profile, Part 3, Specification operators (ISO catalogue record on Genorma)
4. DIN EN ISO 21920-2:2022-12, replaces DIN EN ISO 4287:2010-07 (DIN Media)
5. DIN EN ISO 21920-3:2022-12, replaces DIN EN ISO 4288:1998-04 (DIN Media)
6. ISO 21920-1:2021 replaces ISO 1302:2002 (SIS, Swedish Standards Institute)
7. DIN 4766-1:1981-03, attainable Rz by manufacturing method, withdrawn (DIN Media)
8. Profile parameters from ISO 4287 (Digital Surf Surface Metrology Guide)
9. ISO 21920 parameters, what changed against ISO 4287 (Digital Surf Surface Metrology Guide)
10. What are the differences between ISO 4287 and ISO 21920 (Digital Surf)
11. Roughness evaluation according to ISO 21920, setting classes Sc1 to Sc5 (Polytec guideline)
12. Evolving roughness standards: understanding and implementing ISO 21920 (CMM Quarterly)

Questions on a drawing? sales@ertas.engineering

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