Surface Roughness: Ra, Rz and the New ISO 21920

"Ra 3.2" on a drawing only tells an inspector something useful once it's clear which standard that value was measured under. That underlying calculation changed in 2022, yet older drawing templates from the Far East often still reference the previous version without saying so explicitly.

In brief: Surface roughness describes the microstructure of a part's surface and is usually specified through the parameters Ra (arithmetic mean roughness) and Rz (mean roughness depth). Since 2022, the standard ISO 21920 has consolidated the previously separate standards ISO 1302, ISO 4287, and ISO 4288, and it also changes how Rz is calculated. There's no exact conversion between Ra and Rz, only rough rules of thumb.

What Is Surface Roughness, and Why Does It Matter for Buyers?

Surface roughness describes the fine, usually irregular deviations of a part's surface from an ideal smooth plane. This microstructure results from the manufacturing process used, for example tool marks from milling or the grain pattern left by grinding.

For buyers, the callout matters. Alongside general tolerances, it's one of the most common drawing specifications that determines a part's fit, wear behavior, and leak-tightness. A surface specified too coarse can compromise function, while an unnecessarily fine surface drives up machining cost without real benefit.

This is worth a closer look particularly when sourcing from the Far East. If the drawing omits the specification entirely, the foundry or machine shop usually picks whichever surface is most convenient for their own process. That's rarely the one the part actually needs.

What Do the Parameters Ra and Rz Mean?

Ra and Rz are the two most common parameters used to describe roughness under ISO 21920-2, but they measure different properties of the profile. Ra is the arithmetic mean of all deviations from the centerline over the measured length, while Rz is the mean roughness depth averaged across several individual sampling lengths.

The same surface can therefore show a noticeably different Rz value at an identical Ra, depending on how regular the roughness profile is. For functionally critical surfaces, both are often specified together: an Ra value for the average finish and an Rz value as an upper limit on individual, deeper grooves.

How Do You Read Surface Roughness Symbols on Technical Drawings?

Surface roughness is specified on drawings through a basic symbol: an open checkmark shape. The required Ra or Rz value, and optionally the manufacturing process, get added to it. Additional lines on the symbol indicate whether material removal is required, prohibited, or not permitted.

Since the new ISO 21920 standard, there's a further distinguishing feature. A symbol with an added line above the checkmark shows that the callout follows the new standard rather than the older version. Similar to thread tolerances, where abbreviations can differ depending on the standard revision, it's worth a second look at the symbols used on older drawing templates.

Beyond the Ra or Rz value itself, the symbol can also carry the desired manufacturing process, the lay direction of the grooves, and a machining allowance. In practice, though, a plain Ra or Rz callout without these extra symbols is enough for most parts.

What Has Changed Since 2022 With the ISO 21920 Standard?

The ISO 21920 standard has, since 2022, consolidated the previously separate standards ISO 1302 (drawing indications), ISO 4287 (parameter definitions), and ISO 4288 (evaluation rules) into one series. For buyers, that's more than a purely administrative simplification.

In our sourcing projects, we repeatedly find that drawings from Far East manufacturers are still dimensioned under the old standard without any explicit note to that effect. That matters because the calculation of Rz has also changed under the new standard. Rz is still averaged across multiple sections, but the rules for how those sections are formed have changed under the new standard. That can produce a different number for an identical physical surface than the old evaluation method would. The default acceptance rule changed too: instead of the previous 16-percent rule, ISO 21920 now defaults to the so-called Tmax rule (max-value rule).

Can Ra Be Converted to Rz?

There's no exact conversion between Ra and Rz, since the two parameters capture different statistical properties of the roughness profile. A common rule of thumb holds that Rz is roughly four to seven times Ra. That factor actually ranges from about three to ten times, though, depending on the manufacturing process and profile shape.

Anyone who needs both values on a binding basis, for example for a functionally critical sealing surface, should have them measured directly rather than relying on a conversion. This is worth double-checking especially for parts from Far East production, where often only one of the two values is documented in the inspection report. Ask for the other parameter before rejecting a part as nonconforming.

What N-Grades Exist, and Are They Still Current?

Older drawings often still specify surface roughness through so-called N-grades, a classification into twelve roughness grades under the now-superseded ISO 1302. Each grade corresponds to a fixed Ra value.

Under ISO 21920, N-grades are considered obsolete; new drawings specify the Ra or Rz value directly instead. In practice, though, buyers still encounter N-grades regularly, especially on older drawing templates from Far East production.

Which Surface Roughness Fits Which Manufacturing Process?

The roughness a manufacturing process achieves depends heavily on the tool, feed rate, and material being machined, so only rough tendencies can be given here. Roughing processes like sawing or coarse milling produce comparatively rough surfaces, while grinding and lapping produce very fine ones.

In our sourcing projects with stainless steel parts, we often see customers specify a noticeably finer surface than the part's function actually requires, purely for appearance. That drives up machining cost unnecessarily, which is why we recommend treating functional and cosmetic requirements separately.

How Does Surface Roughness Relate to Other Tolerance Specifications?

Surface roughness never stands alone on a drawing; it complements dimensional, form, and position tolerances. A tight H7 fit, for example, usually assumes a defined surface finish in practice too, since otherwise the friction pairing won't behave as calculated.

Something similar applies to form and position tolerances. A tight roundness tolerance isn't worth much if the surface itself stays too rough to hold the required fit over time. Aligning all three specifications, dimension, form/position, and surface, with the foundry or machine shop up front avoids rework at incoming inspection.

That matters especially for parts that combine several functionally critical surfaces, for instance a fitted bore next to an adjacent sealing face. It isn't enough to carefully specify just one of the three values while leaving the other two open.

Frequently Asked Questions About Surface Roughness

What Is the Difference Between Ra and Rz?

Ra is the arithmetic mean of all deviations from the centerline over the measured length, while Rz is the mean roughness depth from several individual sections. Both describe the same surface from a different statistical angle and can't be converted into each other exactly.

Is DIN EN ISO 1302 Still Valid?

DIN EN ISO 1302 was superseded by the ISO 21920 series in 2022. Older drawings referencing ISO 1302 remain valid, but new drawings should be switched over to the current standard.

What Does N7 or N8 Mean for Surface Roughness?

N7 corresponds to an Ra value of 1.6 µm, N8 to an Ra value of 3.2 µm. Both designations come from the now-outdated N-grade classification and are replaced by direct Ra or Rz callouts on new drawings.

What Roughness Does a Sealing Surface Need?

A blanket figure can't be stated responsibly, since the roughness a seal needs depends heavily on the seal type and operating pressure. In practice, it's best to set the value together with the seal manufacturer or the foundry rather than relying on general rules of thumb.

Conclusion: Surface Roughness Is Worth Specifying Precisely on the Drawing

Specifying Ra or Rz together with the current standard clearly on the drawing avoids confusion between the old and new evaluation methods. It also avoids unnecessarily expensive rework. Especially for drawings from Far East production, that extra look at the symbols used pays off.

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