Ra (arithmetic mean) or Rz (maximum height)? The honest answer depends on four things: the load the part carries, the environment it lives in, the tolerance the drawing calls for, and the volume you are buying. Below is the comparison we use when we quote both options.
If you already know which one you need, send the drawing and we will confirm it. If you are not sure, the table below usually settles it in one pass.
Ra vs Rz Surface Finish: side-by-side comparison
| Dimension | Ra (arithmetic mean) | Rz (maximum height) |
|---|---|---|
| Cost | Baseline spec, easier to achieve | Stricter in practice |
| Strength & performance | Average roughness, hides peaks | Peak-to-valley, governs sealing |
| Corrosion / durability | Averages out the profile | Captures the worst valleys where corrosion starts |
| Machinability & tolerance | Typical Ra 3.2 down to 0.4 um | Usually 4-7x the Ra value |
| Lead time & tooling | Standard tooling | May need lapping or fine grinding |
| Choose it when | The general finish level matters | Sealing, fatigue or coating adhesion matters |
How to decide
- Start with the failure mode, not the price. If the part fails from corrosion, Ra (arithmetic mean) and Rz (maximum height) are not interchangeable no matter what the price difference is.
- Check the tolerance the function actually needs. Tightening tolerance on a material that machines poorly multiplies the cost difference.
- Check availability. A cheaper material with a four-week lead time is more expensive than a dearer one that ships in a week.
- Check the finishing route. Plating and anodizing behave differently on each option and can change the decision on their own.
Where the cost difference comes from
Material price is only part of it. Machining time, tool wear, scrap rate and finishing cost usually decide the final number. A material that costs twice as much but machines three times faster can be cheaper per part. That is why we quote both options with the process route shown, not just a single price.
Frequently asked questions
Is Ra (arithmetic mean) better than Rz (maximum height)?
Not in general. Ra (arithmetic mean) wins when the general finish level matters, and Rz (maximum height) wins when sealing, fatigue or coating adhesion matters. Give us the drawing and we will tell you which one we would choose and why.
How much more does Ra (arithmetic mean) cost than Rz (maximum height)?
The material cost difference is only one input. Machining time and tool life matter as much. Send the part and we will quote both options side by side so the comparison is real.
Can you machine both Ra (arithmetic mean) and Rz (maximum height)?
Yes. Both are routine for us, and we will say so if one of them is a poor fit for your geometry rather than quote it anyway.
Which one should I use for an export product?
Availability and documentation usually decide it. Both are widely stocked and certified, so the tie-breaker is normally the end user’s specification or the applicable standard.
Related pages
- Comparisons — the full index for this section.
- IT6 vs IT7: Tolerance Grade Selection
- Metric vs Imperial Threads for Export Parts
- 5-Axis vs 3-Axis: Real Cost Comparison
- H7 vs H8: Hole Basis Fit Selection
- Instant CNC quote
- Tolerance quick reference
- Frequently asked questions
Get a price for your part
Send a STEP file and a drawing with tolerances, material and quantity. We quote within one working day and machine in our own facility in Shenzhen — milling, turning, EDM, grinding and finishing under one roof, no trading middleman.
Start an instant quote or send your drawings directly. English-speaking engineering support, DFM feedback before you commit to a batch.