Cpk Calculator
Instantly calculate Process Capability Index (Cpk) from Mean, Sigma, USL, and LSL.
Process Capability Index (Cpk) Calculator
Introduction
For manufacturing engineers who already pull mean and sigma off an SPC chart or CMM run and need the capability index checked against a drawing's upper and lower limits before a PPAP or first-article sign-off — this calculator evaluates standard Cpk by finding the minimum of (USL-Mean)/(3*Sigma) and (Mean-LSL)/(3*Sigma) to verify strict six-sigma manufacturing compliance, so the result reflects strict six-sigma manufacturing compliance and the worse of the two sides, not an average of both.
How to Use This Cpk Calculator
- Enter the process mean in the same unit as your specification limits.
- Enter the process standard deviation (1σ) in that same unit.
- Enter the upper specification limit (USL) from the drawing.
- Enter the lower specification limit (LSL) from the drawing.
- Read Cpk in large type and check the Cpu and Cpl badges below it — the smaller of the two is the one that sets your Cpk.
FAQ
Q: How is Cpk calculated from mean, sigma, USL and LSL?
A: Cpu = (USL − Mean) ÷ (3 × Sigma) and Cpl = (Mean − LSL) ÷ (3 × Sigma); Cpk is the minimum of those two, so it always reflects the specification limit the process is closest to.
Q: What happens if I enter a sigma of zero?
A: Both Cpu and Cpl divide by 3 × Sigma, so a zero sigma produces infinite capability — the tool reports that and flags it, because zero variation in a sample usually means a data-entry or measurement-resolution problem.
Q: Why are my Cpu and Cpl different?
A: Because the process mean is not centred between USL and LSL — one side has more room than the other, and Cpk takes the smaller value, which is the side with less room.
Q: Do the mean, sigma, USL and LSL have to be in the same units?
A: Yes. Cpu and Cpl subtract the mean from a limit and divide by 3 × Sigma, so mixing units (millimetres for the limits, micrometres for sigma, for example) returns a meaningless index.