Quick answer: Ra, Rz and Rt all describe surface roughness but measure different things. Ra is the arithmetic average of all height deviations from the mean line, stable and best for general comparison. Rz is the average maximum peak-to-valley height over several sampling lengths, more sensitive to individual peaks and valleys, preferred where the worst features matter. Rt is the single largest peak-to-valley distance over the whole evaluation length, capturing the absolute worst defect. Use Ra for general finish control, Rz when occasional scratches or peaks are critical, and Rt for sealing and fatigue surfaces where one flaw can cause failure.
Designers and machinists often treat all roughness numbers as interchangeable, which leads to wrong targets and failed inspections. Ra, Rz and Rt answer different questions about the same surface, and a part can pass on one while failing on another. This guide defines each parameter, explains how they differ, shows when to use which, and clears up the persistent myth that you can simply convert between them.
It builds on surface roughness Ra explained and the lookup values in the surface roughness chart, within the framework of what is surface finish in CNC machining.
Defining Each Parameter
Ra — Arithmetic Average Roughness
Ra averages the absolute distances of every profile point from the mean line over the sampling length. Because it averages everything, it is stable, repeatable and forgiving of outliers, which makes it the world's default parameter for general finish control. Its weakness is that it can hide a few deep defects in the average.
Rz — Average Maximum Height
Rz divides the evaluation length into several sampling lengths, finds the largest peak-to-valley height in each, and averages those maxima. This makes Rz far more sensitive to the prominent features of a surface than Ra. A surface with occasional deep scratches will show a much higher Rz than its Ra suggests, which is exactly why automotive and many European specs favor Rz.
Rt — Total Height of the Profile
Rt is the distance from the highest peak to the deepest valley across the entire evaluation length, a single worst-case number. It does no averaging at all, so it captures the one defect that could leak, crack or seize. Rt is always equal to or greater than Rz.
Image suggestion 1 — One surface profile annotated three ways, showing how Ra, Rz and Rt each derive a different number from the same trace.
Alt text: "One surface profile showing how Ra, Rz and Rt produce different roughness values."
Placement: under this section.
How They Differ in Practice
The key difference is sensitivity to extremes. Consider two surfaces:
- Surface A is uniform with many small, even peaks and no large defects.
- Surface B is mostly smooth but has a few deep scratches.
These two can show nearly identical Ra, because Ra averages everything and the few scratches barely move the mean. But Surface B will show a much higher Rz and a dramatically higher Rt, because those parameters latch onto the deep scratches. If you only specified Ra, you would accept both surfaces as equal, even though Surface B might leak across a seal or initiate a fatigue crack at one of those scratches.
This is the central lesson: Ra tells you about the typical texture, while Rz and Rt tell you about the worst features. Choose the parameter that matches what actually causes failure on your part.
| Parameter | What it captures | Sensitive to single defects | Typical use |
|---|---|---|---|
| Ra | Average texture | Low | General finish control, cosmetic comparison |
| Rz | Average of worst peaks/valleys | Medium-high | Automotive, sealing, fatigue-aware specs |
| Rt | Single worst peak-to-valley | Highest | Critical sealing, fatigue, leak-tight surfaces |
When to Use Each Parameter
- Use Ra for the majority of machined surfaces, cosmetic faces and general quality control, where you want a stable, comparable measure of overall texture. It is the right default for smartphone frames and general aluminum parts.
- Use Rz when individual peaks or valleys matter, such as coating adhesion (anodizing keys into the profile, see aluminum anodizing guide), sliding contact, or where a customer specification follows automotive convention.
- Use Rt for sealing faces, hydraulic surfaces and fatigue-critical features where a single deep valley is a failure risk. Rt is the strictest gate.
Many demanding drawings specify two parameters together, for example Ra 0.8 and Rz 4, to control both the typical texture and the worst feature at once.
Why You Cannot Reliably Convert Between Them
The most common error is measuring Ra and converting to Rz with a fixed multiplier. The ratio of Rz to Ra depends on the shape of the surface profile, which differs by process:
- A finely turned surface with regular, periodic peaks has a fairly predictable Rz-to-Ra ratio.
- A ground or bead-blasted surface with random texture has a different ratio.
- A surface with occasional defects breaks any fixed ratio entirely, because the defects drive Rz and Rt but not Ra.
A rule of thumb says Rz is roughly four times Ra, but real ratios range from about three to nearly ten. Therefore, if a drawing specifies Rz, measure Rz directly; never measure Ra and convert. The surface roughness chart lists approximate equivalents for planning only, not for certification.
Image suggestion 2 — Two surfaces with equal Ra but very different Rz and Rt, side by side, to show why conversion fails.
Alt text: "Two surfaces with equal Ra but different Rz and Rt showing why roughness conversion is unreliable."
Placement: end of this section.
How Machining Affects Each Parameter
The same machining levers improve all three parameters, but the worst-case parameters are more demanding to satisfy. Reducing feed per tooth and using a sharp tool with a generous nose radius lowers Ra and Rz together. But hitting a tight Rt or Rz also requires eliminating the occasional defect: a chip that gets recut, a momentary vibration, a built-up edge fragment that tears the surface. That means consistent chip evacuation, a rigid vibration-free machine and fresh tooling matter even more for Rz and Rt than for Ra. A high-speed, rigid machine that suppresses built-up edge and chatter is what lets you hold the worst-case parameters, not just the average; see best CNC machine for surface finish and how to measure surface roughness.
Recommended HYR Machines
- HYR VMC850 — high-speed spindle suppresses built-up edge, helping hold tight Rz and Rt as well as Ra.
- HYR VMC1060 — rigid, stable cutting that limits the defects driving worst-case parameters.
- HYR 5 Axis Machining Center — short-tool stability for consistent finish on contoured surfaces.
Need to hold a specific Ra, Rz or Rt? Use the HYR Machine Selector to match spindle speed and rigidity to your parameter and target.
Frequently Asked Questions
What is the difference between Ra and Rz?
Ra is the arithmetic average of all height deviations from the mean line, while Rz averages the largest peak-to-valley heights over several sampling lengths. Rz is more sensitive to individual peaks and valleys, so it catches defects that Ra averages away.
What does Rt measure?
Rt is the total height of the profile, the single largest distance from the highest peak to the deepest valley over the whole evaluation length. It captures the absolute worst defect and is always equal to or greater than Rz.
When should I use Rz instead of Ra?
Use Rz when individual peaks or valleys matter, such as coating adhesion, sliding contact, sealing, or when a customer specification follows automotive convention. Ra remains the default for general finish control.
Can I convert Ra to Rz?
Not reliably. The ratio depends on the surface profile shape and ranges from about three to nearly ten. A rule of thumb of four is for rough planning only. If a drawing specifies Rz, measure Rz directly rather than converting from Ra.
Why can two surfaces have the same Ra but different Rz?
Because Ra averages all deviations while Rz captures the worst peaks and valleys. A surface with a few deep scratches can have the same average roughness as a uniform surface but a much higher Rz and Rt.
Should I specify both Ra and Rz on a drawing?
For demanding parts, yes. Specifying both, for example Ra 0.8 and Rz 4, controls the typical texture and the worst feature at once, which is common on sealing, fatigue and coating-critical surfaces.
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