How to Calculate Cp and Cpk
Cp compares specification width with short-term process spread. Cpk adds the process mean, so it identifies the weaker side of an off-centre process. Calculate them only after confirming that data, subgrouping and stability are suitable.
Technical publication review required before indexing · Draft reviewed 13 August 2026
Engineering workflow
- Confirm LSL and USL are engineering specifications, not control limits.
- Preserve time order and investigate special-cause signals.
- Document the within-process sigma estimator.
- Calculate Cp for potential spread and Cpk for achieved centring.
- Compare the result with the requirement defined by the customer or control plan.
Worked example
With LSL 9.95 mm, USL 10.05 mm, mean 10.01 mm and within sigma 0.01 mm: Cp = 0.10/(6×0.01) = 1.667. Cpu = 0.04/(3×0.01) = 1.333 and Cpl = 0.06/(3×0.01) = 2.000, so Cpk = 1.333.
Run the Cp Cpk calculatorLimitations and checks
- Capability is not meaningful for an unstable process.
- A normal-model index can mislead for skewed, bounded or multimodal data.
- A point estimate does not show confidence interval uncertainty.
Relevant AccuTouch next step
Use the calculation as a screening input, then confirm the actual gauge, fixture, software or automation configuration.
Connect readings to AccuSPC