Surface Roughness Ra to Rz Calculator
Convert surface roughness values between Ra (Arithmetic Average Roughness) and Rz (Maximum Height of Profile) instantly. This calculator is essential for CNC machining, precision engineering, and quality inspection where surface finish specifications must meet international standards.
Ra To Rz
Calculator
Quick Answer
- Ra to Rz (approx.): Rz ≈ Ra × 7
- Rz to Ra (approx.): Ra ≈ Rz ÷ 7
👉 These are commonly used engineering approximations for quick conversion.
Examples:
- Ra 1.6 µm ≈ Rz 11.2 µm
- Ra 3.2 µm ≈ Rz 22.4 µm
- Rz 40 µm ≈ Ra 5.7 µm
Formula
Ra to Rz:
Rz approx Ra times 7
Rz to Ra:
Ra approx frac
👉 Note:
This is an approximate conversion. Actual values may vary depending on surface profile, machining process, and measurement standard (ISO vs JIS vs DIN).
Ra to Rz Conversion Table
| Ra (µm) | Approx. Rz (µm) |
|---|---|
| 0.2 | 1.4 |
| 0.4 | 2.8 |
| 0.8 | 5.6 |
| 1.6 | 11.2 |
| 3.2 | 22.4 |
| 6.3 | 44.1 |
| 12.5 | 87.5 |
| 25 | 175 |
👉 Quick reference:
- Fine finish → Ra < 1.6 µm
- Standard machining → Ra 1.6 – 3.2 µm
- Rough machining → Ra > 6.3 µm
How to Use This Calculator
- Enter a value in Ra (µm) or Rz (µm)
- The calculator automatically converts the value
- Results update instantly
- Click Copy to save results
- Click Reset to clear inputs
✔ Bi-directional conversion
✔ Real-time calculation
✔ Suitable for machining and inspection
Engineering Insight
Surface roughness directly affects part performance, function, and cost.
Ra vs Rz — what's the difference?
- Ra (Average Roughness):
Measures the average surface deviation
→ Most commonly used in engineering drawings - Rz (Peak-to-Valley Height):
Measures the maximum height difference
→ More sensitive to surface defects
Why conversion matters:
- Different countries use different standards
(ISO → Ra, DIN/JIS → often Rz) - Customers may specify roughness differently
- CNC machining and grinding processes produce different surface profiles
Practical impact in machining:
- Lower Ra → smoother surface → higher cost
- Higher Ra → rougher surface → faster machining
👉 Example:
- Ra 0.8 µm → requires fine finishing or grinding
- Ra 3.2 µm → achievable by standard CNC machining
💡 Tip:
Always confirm whether the drawing specifies Ra or Rz before production.
Common Engineering Uses
CNC Machining
Define surface finish requirements for parts.
Grinding & Finishing
Control surface quality for high-precision components.
Quality Inspection
Convert between Ra and Rz for different measurement systems.
Aerospace & Automotive
Critical for sealing surfaces, bearings, and sliding parts.
Supplier Communication
Avoid misunderstandings between different roughness standards.
Related Calculators
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👉 Combine these tools to optimize machining parameters and surface quality.
FAQ
What is Ra in surface roughness?
Ra is the average roughness of a surface, representing the mean deviation from the centerline.
What is Rz?
Rz measures the maximum peak-to-valley height within a sampling length.
Is Ra × 7 always accurate?
No. It is an approximation. Actual conversion depends on machining method and surface characteristics.
Which is more important, Ra or Rz?
- Ra → general engineering use
- Rz → critical surfaces with peak sensitivity
What Ra value is considered smooth?
- Ra ≤ 1.6 µm → smooth
- Ra ≤ 0.8 µm → very smooth
- Ra ≤ 0.4 µm → precision finish
