Ra vs Rz
Understand the difference between Ra and Rz surface roughness values.
Ra and Rz are both used to describe surface roughness, but they measure different characteristics of a surface profile.
What Is Ra?
Ra is the arithmetic average roughness value. It represents the average deviation of a surface profile from the mean line.
What Is Rz?
Rz is commonly used to describe average peak-to-valley height over sampling lengths. It is more sensitive to peaks and valleys than Ra.
Ra vs Rz Comparison
| Item | Ra | Rz |
|---|---|---|
| Meaning | Average roughness | Average peak-to-valley height |
| Sensitivity | Less sensitive to isolated peaks | More sensitive to peaks and valleys |
| Common use | General surface finish reference | Functional surface control |
| Exact conversion | No | No |
Can Ra Be Converted to Rz Exactly?
No. Ra and Rz cannot be converted exactly because they describe different surface roughness characteristics. Any conversion should be treated as an estimate.
FAQ
Can Ra be converted directly to Rz?▼
No. Ra and Rz describe different surface roughness characteristics. This calculator provides an estimated value only.
What is the common Ra to Rz formula?▼
A common engineering estimate is Rz ≈ Ra × 5. A broader reference range is Rz ≈ Ra × 4 to 7.
Is Rz always 5 times Ra?▼
No. The ratio depends on surface profile, machining process, material, cutoff length, and measurement method.
How do I convert Rz to Ra?▼
Use the Rz→Ra calculator or bidirectional mode. A common engineering estimate is Ra ≈ Rz ÷ 5, with a broader reference range of Ra ≈ Rz ÷ 7 to Rz ÷ 4. Results are approximate only.