What Ra and Rz Mean in Surface Roughness

What Is Surface Roughness?

Every manufactured surface carries a texture left by the process that made it: the scratches of an abrasive belt, the feed marks of a cutter, the striations on a laser-cut face. Surface roughness is the fine, closely spaced part of that texture. Standards split surface texture into components by wavelength:

• Roughness: short-wavelength peaks and valleys produced directly by the tool or abrasive.
• Waviness: longer, wider-spaced undulations from vibration, deflection or heat.
• Form: the overall shape of the part, such as flatness or straightness. On sheet and plate, form errors are corrected by roller leveling, not by finishing.
• Lay: the direction of the dominant surface pattern, for example the grain of a brushed finish.

A roughness instrument traces a line across the surface and records a height profile. Filters then separate roughness from waviness. The key filter setting is the cutoff wavelength, written λc: undulations longer than the cutoff are treated as waviness and removed from the roughness profile. Parameters such as Ra and Rz are calculated from what remains.

On sheet metal parts, roughness matters for more than looks. It affects how a coating or adhesive grips, how easily a food-contact surface cleans, how a part slides or seals, and how a visible panel reflects light. That is why a finishing requirement on a drawing is usually a roughness value plus a direction, not just a grit number. TFON's page on sheet metal surface finishing covers how those requirements are met in production.

What Is Ra?

Ra is the arithmetic mean deviation of the roughness profile. Take every point of the profile, measure its height above or below the mean line, ignore the sign, and average those values over the measured length. Written as a formula, Ra = (1/l) × ∫ |Z(x)| dx, where Z(x) is the profile height and l is the length over which it is evaluated.

The definition is the same in ISO and US practice. ISO 4287 defined Ra for many years; ISO 21920-2, published in December 2021, has replaced it. In the United States, ASME B46.1 calls the parameter roughness average and uses the same arithmetic-average definition. What differs is the unit. ISO drawings give Ra in micrometers (µm), while US drawings usually give it in microinches (µin). One micrometer is about 39.4 microinches, so Ra 0.8 µm is roughly 32 µin, and Ra 3.2 µm is roughly 125 µin.

What Ra tells you, and what it hides

Ra is stable and repeatable, which is why it is the default roughness parameter on most drawings worldwide. Because it averages the whole profile, a single deep scratch or an isolated burr-like peak barely moves it. Two surfaces with very different profiles can share the same Ra: one with sharp peaks, another with sharp valleys. For a painted panel that may not matter. For a sealing face or a thin coating, it can.

Ra also depends on direction. On a directional finish such as a brushed stainless sheet, a trace across the grain reads higher than a trace along it. TFON's finishing guidance puts it plainly: Ra describes an average of profile deviations; it does not describe every aspect of appearance or every isolated scratch.

A related parameter is Rq, the root mean square of the same deviations. Older US drawings sometimes call it RMS. Rq weights large deviations more heavily and reads higher than Ra on the same profile, so the two are not interchangeable.

What Is Rz?

Rz is the maximum height of the roughness profile. Within one sampling length, add the height of the highest peak (Rp) to the depth of the deepest valley (Rv): Rz = Rp + Rv. Under ISO 4287 and its companion ISO 4288, the reported Rz is the mean of the values from five consecutive sampling lengths. ISO 21920-2 kept this approach for Rz: while Ra is now calculated once over the whole evaluation length, peak and valley parameters are still evaluated on individual section lengths so the results stay stable.

ASME B46.1 in the United States works the same way in principle: Rz is the average of the peak-to-valley heights measured in each sampling length.

Three meanings of one symbol

Rz has changed definition over the decades while keeping the same symbol, which causes real disputes between suppliers and customers:

• Current ISO and ASME Rz: mean of the peak-to-valley heights of the sampling lengths, as above. German drawings built on the former DIN 4768 use essentially this definition.
• Ten-point height: the older ISO definition, averaging the five highest peaks and five deepest valleys in one sampling length. Japanese standards keep it under the name RzJIS.
• Single maximum values: Rt, the total height over the full evaluation length, and Rz1max, the largest single peak-to-valley height.

If a drawing says Rz and comes from an older document or another country, confirm which definition is meant before quoting a finishing operation.

Why Rz is specified

Rz responds to the extremes that Ra averages away. Engineers call it out when isolated peaks can break through a thin coating, damage a seal or cause wear on a sliding surface, or when deep valleys can trap dirt or start corrosion. Rz is common on German drawings, so parts for German customers may carry an Rz value where a US drawing would show Ra.

Ra vs Rz: What Is the Difference?

Ra is an average; Rz is a height. Ra says how far the profile deviates from its mean line on average. Rz says how tall the profile is from its highest peaks to its deepest valleys. On any real surface Rz is the larger number, and the gap between the two depends on the shape of the profile.

Converting between Ra and Rz

There is no exact conversion. A common planning estimate is that Rz runs about four to seven times Ra: closer to four on smooth, even surfaces and closer to seven on rough, irregular ones. Some metrology references note that when an Ra requirement is converted into an Rz requirement, the ratio can run well above seven, because a surface that just meets an Ra limit can still carry tall isolated peaks.

Treat any ratio as a rough rule for early planning only. Do not accept or reject parts on a converted value. If the drawing says Rz, measure Rz. If supplier and customer disagree, the practical fix is to measure the same part with the same parameter, filter and cutoff on both sides.

Which one to specify

For most sheet metal appearance and coating work, Ra with a stated direction is enough. Add Rz, or specify it instead, when peaks and valleys matter to function: thin coatings, gaskets, sliding contact or hygiene surfaces where valleys hold residue.

Typical Roughness Values by Process

Every process has a typical roughness band. Hot rolling and flame cutting sit around Ra 12.5-25 µm, cold rolling around Ra 0.8-3.2 µm, grinding around Ra 0.1-1.6 µm, and polishing or lapping below Ra 0.4 µm. These ranges from the production-method chart in Machinery's Handbook are a starting point, not a promise. A laser-cut face varies with material, thickness, assist gas and machine condition, so measure your own parts when a value matters.

The complete table, with Ra in µm and µin, N grades and the stainless steel finishes defined in ASTM A480 and EN 10088-2, is in our surface finish chart. For the finishes themselves (ground, brushed, satin and edge-rounded) and how each is produced, see our guide to types of metal finishes.

Roughness Grades N1 to N12 (ISO 1302)

Older drawings and many shop charts use roughness grade numbers instead of Ra values. The N-grades come from the 1992 edition of ISO 1302. Each step doubles the Ra value, from N1 (Ra 0.025 µm) to N12 (Ra 50 µm), and N7 matches the familiar Ra 1.6 µm or 63 µin callout.

The 2002 edition of ISO 1302 dropped the N-grades, and ISO 1302 itself was withdrawn at the end of 2021 and replaced by ISO 21920-1. New drawings should state the parameter and its value directly, for example Ra 1.6 or Rz 10. The full conversion of N grades, Ra and approximate RMS values is printed in the surface finish chart with RMS values, which is also available as a PDF.

Surface Roughness Symbols on Drawings

A roughness requirement on a drawing is built on a basic symbol: two lines of unequal length forming a check mark that touches the surface or an extension line. Under ISO 1302 and its successor ISO 21920-1, three variants tell the shop how the surface may be made:

• Basic symbol (open): any production method is allowed.
• With a horizontal bar closing the triangle: material removal is required, for example grinding or machining.
• With a circle in the V: material removal is not permitted; the surface stays as rolled, cast or formed.

The requirement itself is written on a horizontal line extending from the long leg, in the form parameter plus limit value, for example Ra 1.6. It can also carry an upper or lower limit (U or L), filter and evaluation length settings, the production method and a lay symbol. The lay symbols are = (parallel to the view plane), ⊥ (perpendicular), X (crossed in two directions), M (multidirectional), C (circular), R (radial) and P (particulate, non-directional).

What changed with ISO 21920-1

ISO 21920-1 introduced a modified symbol so that a drawing shows which rule set applies. A drawing with the new symbol is read under ISO 21920; one with the old symbol is read under ISO 1302 and its companion standards. The default acceptance rule also changed. ISO 1302 used the 16 percent rule, which let up to one in six measured values exceed the limit. ISO 21920-1 makes the max rule the default: no measured value may exceed the limit.

US drawings

In the United States the symbols follow ASME Y14.36, and values are usually given in microinches. On many US drawings a bare number above the check mark, such as 63 or 125, is read as Ra in microinches. When a drawing mixes units or leaves the parameter unstated, ask before quoting.

How Is Surface Roughness Measured?

Stylus profilometers

The workhorse instrument is the contact stylus profilometer. A diamond stylus is drawn across the surface at constant speed and its vertical movement is recorded as a profile. ISO 3274 sets the nominal stylus geometry: a conical tip with a 60° or 90° cone angle and a tip radius of 2, 5 or 10 µm. Portable units handle most shop-floor checks on sheet metal parts; lab instruments add longer traces and finer resolution.

The cutoff is chosen from the expected roughness. For non-periodic profiles, ISO 4288, now replaced by ISO 21920-3, gives the following defaults. The evaluation length is five cutoffs.

Optical methods

Non-contact instruments, such as confocal microscopes, coherence scanning interferometers and focus variation systems, measure an area instead of a line. They report areal parameters like Sa and Sz under ISO 25178-2. They suit soft or delicate surfaces, but results do not always match stylus readings on the same part, so agree on the method before comparing numbers.

Practical rules for sheet metal parts

1. Trace across the lay unless the drawing says otherwise, and record the direction.
2. Measure on a clean, flat area away from edges, holes and handling marks.
3. Record instrument, stylus, cutoff and evaluation length with every result.
4. Use comparator plates only for quick screening, not for acceptance.
5. Check edge radius separately; a profilometer trace on a flat face says nothing about the edge. See sheet metal edge rounding for how edges are specified and inspected.

Abrasive Grit and Ra: What Controls the Finish?

No standard maps a grit number to an Ra value. Finer grit tends to lower Ra, but the result also depends on the abrasive type and condition, belt or brush speed, contact pressure, feed rate, the material and the marks left by earlier steps. A fine final stage does not remove deep scratches from a coarse one; each stage has to take out the marks of the one before. In TFON's own words, a grit number alone does not define a finish. Acceptance needs a reference sample or a measured roughness value.

Grit scales add their own confusion. Coated abrasives are graded under FEPA P-grades in Europe and ANSI/CAMI grades in the United States, and the two scales drift apart at the fine end. When a finish is carried over from one supplier to another, compare samples, not labels.

TFON consumables and their grit ranges

• Abrasive belts in aluminum oxide, ceramic and zirconium: grit 40-400. Cubitron belts: grit 24-400. Belt sizes run from 680x1820 mm to 1630x1820 mm.
• Edge rounding flap brushes: grit 60-400, in Ø270x250, Ø300x400 and Ø350x490 mm sizes.
• Scotch non-woven belts: coarse, medium, fine and very fine, for satin finishing.

On a TFON Surfacer, the belt sets the base scratch pattern and the flap or non-woven stations refine it. The TF-RF-3013 surface finishing machine covers a 1,300 mm working width; the TF-RBF-2510 deburring and finishing machine covers 1,000 mm and adds deburring in the same pass. Both run at 0.6-4.0 m/min and work the top face of the part per pass. The full range of deburring and edge rounding machines shows which stations each model carries.

The reliable way to set a finish is a trial: run your parts, measure Ra before and after on the same instrument and in the same direction, and keep a signed reference sample. TFON has not yet published its own before-and-after roughness data, so qualify the result on your material.