Engineering tool
Cp and Cpk process capability calculator
Calculate capability indices and the expected nonconforming rate for a normally distributed characteristic.
Inputs
With data, the tool calculates the sample mean and overall standard deviation and reports Pp and Ppk.
Result
Enable JavaScript to use this calculator. The method and formulas are explained below.
How to use
Enter the upper and lower specification limits. Either enter the process mean and the within-subgroup standard deviation, or paste individual measurements. With pasted data, the tool reports Pp and Ppk because it uses the overall sample standard deviation.
Formulas
Cp = (USL − LSL) / (6σ)
Cpu = (USL − μ) / (3σ)
Cpl = (μ − LSL) / (3σ)
Cpk = min(Cpu, Cpl)
ppm = [ (1 − Φ((USL − μ)/σ)) + Φ((LSL − μ)/σ) ] × 10⁶
Assumptions
The process is stable (in statistical control) and approximately normal. The measurement system is capable. Results for small samples have wide confidence intervals. See our capability guide.
Use of this toolThis calculator is an educational aid. It is not a substitute for the governing standard, the customer specification or engineering approval. Verify results before using them for acceptance decisions.