Statistics & Probability

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Process Capability Index Calculator.

Calculate Cp, Cpu, Cpl, and Cpk from specification limits, process mean, and standard deviation.

On-device calculationNo signup
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Set your values

Results update as you type.

cp: 1.666667

cp

1.666667
cpu: 1.666667

cpu

1.666667
cpl: 1.666667

cpl

1.666667
cpk: 1.666667

cpk

1.666667

Cp/Cpk are capability indices from entered limits and sigma; this route does not claim normality, control, or product safety.

Results update automatically as you type.

Use Cases

Compare Processes or Machines

Calculate capability indices for different machines or processes to compare their ability to meet specifications. This helps in selecting equipment or identifying improvement priorities.

Example: Compare two machines by entering their respective means and standard deviations with the same spec limits.

Frequently Asked Questions

What is the difference between Cp and Cpk?
Cp measures potential capability assuming the process is centered between the specification limits. Cpk accounts for the actual process mean, indicating how well the process is centered and how close it is to the nearest specification limit. A high Cp with low Cpk suggests the process is off-center.
What does a Cpk value of 1.33 mean?
A Cpk of 1.33 indicates that the process is capable of producing output within specification limits with a relatively low defect rate, typically corresponding to about 4 sigma quality. Many industries use 1.33 as a minimum acceptable threshold.
Can I use this calculator for non-normal data?
This calculator assumes a normal distribution of the process data. For non-normal data, the indices may be misleading. Consider using transformations or alternative capability indices designed for non-normal distributions.

Tips & Common Mistakes

Tips

  • Ensure your process is in statistical control before interpreting capability indices; otherwise, the values may be misleading.
  • Use a sufficiently large sample size (at least 30-50 data points) to estimate the standard deviation reliably.
  • Remember that Cp and Cpk are unitless; they compare the spread and centering of your process to the specification width.
  • If Cpk is much lower than Cp, focus on centering the process to improve capability.

Common Mistakes to Avoid

  • Using the sample standard deviation instead of the process standard deviation; the latter should be based on within-subgroup variation if possible.
  • Assuming that a Cp or Cpk above 1 guarantees zero defects; it only indicates the process is within spec for a normal distribution, but defects can still occur.
  • Ignoring the effect of the process mean: a high Cp with a low Cpk indicates the process is off-center, which is a common oversight.

Last updated: August 13, 2026