Process Capability Index Calculator
Enter specification limits, process mean, and standard deviation to assess Cp and Cpk.
Cp
1.1111
Cpk
1.1111
Cpl
1.1111
Cpu
1.1111
How to Use This Calculator
- Enter lower and upper specification limits (LSL, USL).
- Provide the process mean and standard deviation.
- Review Cp, Cpk, Cpl, and Cpu to assess capability.
- Use interpretation guidance to determine process adequacy.
Formula
Cp = (USL − LSL) / (6σ)
Cpl = (μ − LSL) / (3σ)
Cpu = (USL − μ) / (3σ)
Cpk = min(Cpl, Cpu)
Cp reflects potential capability, while Cpk accounts for mean centering relative to specs.
Full Description
Capability indices evaluate whether a process fits within specification limits. Cp compares spread, while Cpk evaluates both spread and centering.
Processes with Cpk ≥ 1.33 typically meet quality standards, though tolerance depends on industry requirements.
Frequently Asked Questions
Do I need normal distribution?
Capability indices assume roughly normal data. Validate with control charts before applying.
Why is Cp larger than Cpk?
Cp ignores mean centering. If the process mean shifts toward a limit, Cpk decreases while Cp stays the same.
Can I use Cpk for one-sided specs?
Yes. Use Cpl or Cpu alone when only a lower or upper limit is specified.
What if σ is zero?
A zero standard deviation implies no variability; capability indices become undefined or infinite. Verify data for measurement issues.