How to calculate Cpk (process capability)
Cpk is the smaller of (USL − mean) ÷ 3σ and (mean − LSL) ÷ 3σ. It shows capability on the side of the specification the process is closest to.
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The formula
Cp = (USL − LSL) ÷ 6σ; CPU = (USL − μ) ÷ 3σ; CPL = (μ − LSL) ÷ 3σ; Cpk = min(CPU, CPL).
Step by step
- Collect the upper and lower specification limits.
- Estimate the process mean and standard deviation from stable process data.
- Work out CPU and CPL.
- Take the smaller of the two as Cpk.
Worked example
Spec 9.5–10.5, mean 10.1, σ 0.1
- CPU = (10.5 − 10.1) ÷ 0.3 = 1.333
- CPL = (10.1 − 9.5) ÷ 0.3 = 2.000
- Cp = 1.0 ÷ 0.6 = 1.667
Cpk is 1.333; Cp is 1.667, the gap showing the mean sits off-centre.
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APA
Free Calculator. (n.d.). How to calculate Cpk (process capability). https://freecalculator.tech/guides/how-to-calculate-cpk
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