Ra, Rz and Surface Roughness Parameters for Molds
Surface roughness parameters are numbers calculated from a measured surface profile, and the two used most on mold drawings are Ra, the arithmetic mean deviation of the profile, and Rz, the mean peak-to-valley height. Ra averages every point of the profile, so it describes the general level of roughness but hides isolated peaks and scratches. Rz averages the highest peak-to-valley heights over several sampling lengths, so it reacts much more to those extremes. Both are defined for profiles in ISO 21920-2, which replaced ISO 4287, and in ASME B46.1 in the United States. A roughness value is only complete when the parameter, the unit, and the measuring conditions are stated with it.
Surface roughness parameters at a glance
- Ra: arithmetic mean of the absolute profile deviations; the most common parameter on drawings and in mold finish standards.
- Rq: root mean square of the deviations; close to Ra but weights larger deviations more.
- Rz: mean of the maximum peak-to-valley heights of five sampling lengths; sensitive to peaks and scratches.
- Rt: total height of the profile over the whole evaluation length.
- Areal parameters: Sa and Sz describe an area instead of a line, under ISO 25178-2.
- Units: micrometres (µm) in ISO practice, microinches (µin) in much US practice; 1 µm = 39.37 µin.
Why one roughness number is not enough
A roughness parameter compresses a whole profile into one number, and each parameter keeps different information. Two surfaces can have the same Ra and look and perform quite differently: one may be an even field of fine marks, the other a smooth surface with a few deep scratches. Ra barely separates them; Rz and Rt do. This is why mold finish standards and drawings often name a parameter that suits the purpose, and why a cross-reference between two finishes based only on Ra must be treated with care, as the SPI, VDI and Mold-Tech conversion page explains.
The main profile parameters
| Parameter | Name | What it measures | Sensitive to isolated peaks and scratches? | Typical use on molds |
|---|---|---|---|---|
| Ra | Arithmetic mean deviation | Average absolute distance of the profile from its mean line | Low | SPI and VDI 3400 values, general finish callouts |
| Rq | Root mean square deviation | RMS distance of the profile from its mean line | Slightly higher than Ra | Optical and research work; some instrument reports |
| Rz | Maximum height (mean of sampling lengths) | Average of the largest peak-to-valley heights in each sampling length | High | Polished and sealing surfaces, European drawings |
| Rt | Total height of profile | Highest peak to lowest valley over the full evaluation length | Very high | Detecting single deep scratches or pits |
| Rsm | Mean width of profile elements | Average spacing of the profile features | Not a height parameter | Comparing texture spacing, not depth |
Ra in detail
Ra is the average of the absolute heights of the profile above and below its mean line, taken over the evaluation length. It is stable and repeatable, which is why it became the default parameter on drawings and the basis of most mold finish tables. The VDI 3400 scale is defined directly from Ra, and the reference values published for the SPI surface finish grades are approximate Ra values. Its weakness is the same averaging that makes it stable: a few deep scratches on an otherwise smooth surface change Ra very little, even though they are clearly visible on a polished part.
Rz in detail, and why there is more than one Rz
In current ISO practice, Rz is the mean of the maximum peak-to-valley heights found in each of the sampling lengths that make up the evaluation length, usually five. Because it is built from the extremes, Rz rises sharply when a surface carries scratches, pits, or isolated peaks, which makes it useful for polished surfaces where such defects matter.
Rz is also the parameter most often misread, because the same symbol has had different definitions:
- Current ISO Rz: mean of the maximum peak-to-valley heights of the sampling lengths.
- Older "ten-point height" Rz: used in earlier ISO and still in the Japanese JIS standard as Rzjis, based on the five highest peaks and five deepest valleys, which gives different values.
- Rmax and Rz1max: the single largest peak-to-valley height among the sampling lengths, used in some older German and current ISO practice to catch the worst feature.
When a drawing or a supplier report shows Rz, confirm which definition and which standard is meant before comparing values.
Ra and Rz are not convertible
Rz is always larger than Ra for the same surface, but there is no fixed conversion factor between them. The ratio depends on the shape of the profile: an even, regular surface gives one ratio, and a surface with a few deep scratches gives a much higher one. Conversion tables that show a single factor are approximations for one kind of surface and should not be used to turn an Ra callout into an Rz requirement. If both matter, specify both.
Measuring conditions: cutoff and evaluation length
A roughness value depends on how it was measured. A stylus profilometer, the usual instrument, traces a line across the surface, and the result is filtered to separate roughness from longer waves. The filter cutoff, written λc, and the evaluation length strongly affect the result, so two instruments set differently can report different values on the same surface.
| Expected Ra (µm) | Cutoff λc (mm) | Evaluation length (mm) | Typical mold surfaces |
|---|---|---|---|
| Over 0.006 to 0.02 | 0.08 | 0.4 | SPI A-1 and optical mirror polish |
| Over 0.02 to 0.1 | 0.25 | 1.25 | SPI A-2, A-3, B-1 |
| Over 0.1 to 2 | 0.8 | 4 | SPI B-2, B-3, C, D-1, D-2; VDI 12 to VDI 26 |
| Over 2 to 10 | 2.5 | 12.5 | SPI D-3; VDI 27 to VDI 40 |
| Over 10 to 80 | 8 | 40 | VDI 41 to VDI 45 and coarser |
The table is the selection rule from ISO 4288, which many shops and instruments still use. ISO 21920-3 now covers measurement conditions for profiles and sets its own default rules, so the drawing or the agreement with the supplier should state which standard and which cutoff apply. The evaluation length shown is five cutoffs, the traditional default.
Other practical points: measure across the lay on directional finishes such as SPI B and C, because a trace along the polishing lines reads much lower; keep the stylus away from edges; and take several traces and report the average.
Roughness parameters on textured surfaces
Ra and Rz work well for polished, stoned, blasted, and EDM surfaces, where the roughness is fairly uniform. They work poorly for decorative textures such as leather grains, where the pattern is far larger than a roughness sampling length and the relevant figure is the texture depth. Those textures are specified by pattern code, depth, and gloss, not by Ra; see leather grain texture. For any surface, Ra describes the cavity, not how glossy the part will look; for that, see gloss levels and gloss units.
Areal parameters: Sa and Sz
A profile parameter describes one line across the surface. Areal parameters, defined in ISO 25178-2, describe a whole area measured with an optical or contact instrument. Sa is the areal counterpart of Ra and Sz the areal counterpart of the maximum height. Because they capture the surface in all directions, areal parameters describe textured and directional surfaces more completely than a single trace, and they are increasingly used where optical measuring equipment is available. Sa and Ra values on the same surface are similar in concept but not identical, so they should not be mixed in one specification.
Roughness grade numbers
Older drawings, and some current ones, give roughness as an N number instead of a value. Each N grade corresponds to a maximum Ra, doubling from one grade to the next.
| Grade | Ra (µm) | Ra (µin) |
|---|---|---|
| N1 | 0.025 | 1 |
| N2 | 0.05 | 2 |
| N3 | 0.1 | 4 |
| N4 | 0.2 | 8 |
| N5 | 0.4 | 16 |
| N6 | 0.8 | 32 |
| N7 | 1.6 | 63 |
| N8 | 3.2 | 125 |
| N9 | 6.3 | 250 |
| N10 | 12.5 | 500 |
| N11 | 25 | 1000 |
| N12 | 50 | 2000 |
The microinch column uses the rounded values customary in US practice. As a reference point, N5 at 0.4 µm equals VDI 12, the finest grade of the VDI 3400 scale.
How to write roughness on a mold drawing
- Parameter and value: for example "Ra 0.4 µm" or "Rz 2.5 µm", never a bare number.
- Unit: state µm or µin; a value of 16 means very different things in each.
- Limit type: say whether the value is a maximum, a range, or a nominal target.
- Standard: name the standard the symbol and parameter follow, such as ISO 1302 or ISO 21920-1 for indication, or ASME Y14.36 and B46.1 in US practice.
- Measuring conditions: cutoff and direction where they matter, especially across the lay on directional finishes.
- Finish callout: where the surface is a mold finish, give the SPI or VDI grade as well, since roughness alone does not define the method or appearance.
Frequently asked questions
What is the difference between Ra and Rz?
Ra is the average absolute deviation of the profile from its mean line. Rz is the average of the largest peak-to-valley heights in each sampling length, so it reacts much more to scratches and isolated peaks.
Can Ra be converted to Rz?
No. Rz is always larger than Ra, but the ratio depends on the profile shape. Single conversion factors are rough approximations and should not be used for specifications.
Which standard defines Ra and Rz?
ISO 21920-2 defines profile parameters and replaced ISO 4287. In the United States, ASME B46.1 covers surface texture and its parameters.
What cutoff should be used to measure Ra?
It depends on the expected roughness. Under the long-established ISO 4288 table, Ra over 0.1 to 2 µm uses a 0.8 mm cutoff; finer surfaces use 0.25 or 0.08 mm and coarser ones 2.5 or 8 mm.
How do I convert microinches to micrometres?
Divide microinches by 39.37 to get micrometres, or multiply micrometres by 39.37 to get microinches. For example, 16 µin is about 0.4 µm.
What is Sa?
Sa is the areal arithmetic mean height defined in ISO 25178-2, the areal counterpart of Ra. It describes an area rather than a single line.
Why does my roughness reading change with direction?
On directional finishes such as paper polished or stoned surfaces, a trace across the lay crosses the scratches and reads higher than a trace along them. Measure across the lay unless agreed otherwise.
What Ra is VDI 12?
VDI 12 corresponds to Ra 0.40 µm, which is also roughness grade N5. It is the finest grade on the VDI 3400 scale.
Normative References & Sources
- ISO 21920-2, Geometrical product specifications (GPS), surface texture: profile, terms, definitions and surface texture parameters: iso.org
- ISO 25178-2, Geometrical product specifications (GPS), surface texture: areal, terms, definitions and surface texture parameters: iso.org
- ASME B46.1, Surface Texture (Surface Roughness, Waviness, and Lay): asme.org
- ISO 1302, indication of surface texture in technical product documentation: iso.org
Written and technically reviewed by Marcel Dias, Technical Editor, with over 28 years in mold texturing. Parameter definitions are summarized; the governing text is the current edition of the cited standards. Confirm the applicable standard and measuring conditions with your customer before production. See our Disclaimer. Part of the Mold Texture Standards section. Last updated: October 9, 2026.