Comprehensive Calculator Guide
This calculator works out the process capability indexes Cp and Cpk from the specification limits, the process mean and its standard deviation, showing whether a line can meet the specification consistently. A 10 ± 0.5 mm part made at mean 10.1 with SD 0.12 has Cp 1.39 and Cpk 1.11.
Cp and Cpk
Cp compares tolerance with spread; Cpk adds where the mean sits. Cpk below 1 means many defects; 1.33 is a common minimum and 1.67 in automotive. Compute them from a stable process with enough data.
How to Use
- Enter the specification limits.
- Enter the process mean and SD.
- Press “Calculate” to see Cp, Cpk and expected defects.
Formula Used
Cp = (USL − LSL) ÷ 6σ | Cpk = min(USL − μ, μ − LSL) ÷ 3σ
Practical Examples
- Example 1
- Cp 1.39, Cpk 1.11: off-centre.
- Example 2
- Cp = Cpk = 0.83: not capable.
Important Tips
- Re-centre when Cpk ≪ Cp.
- Cut spread when Cp is low.
- Use 100+ measurements.
Common Mistakes to Avoid
- Relying on Cp alone.
- Unstable processes.
- Using control limits.
Frequently Asked Questions
- Cp vs Cpk?
- Centred vs actual.
- Acceptable Cpk?
- 1.33+.
- Below 1?
- Many defects.