Process Capability Calculator (Cp, Cpk)
Enter summary statistics or paste a raw dataset to compute Cp, Cpu, Cpl, and Cpk against your specification limits, plus the estimated out-of-spec parts per million. Built for pharmaceutical and device QA process validation.
Use summary statistics if you already have the mean and standard deviation, or paste raw measurements to compute them.
The process average of your measurements.
The sample standard deviation (n minus 1 denominator).
The tool computes the mean and sample standard deviation (n minus 1) from these values.
Leave blank for a one-sided lower specification.
Leave blank for a one-sided upper specification.
Result
Cpk verdict
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Cp
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Cpu
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Cpl
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Estimated out of spec
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Process summary
Enter your values and select an input mode, then calculate.
Disclaimer: Process capability assumes a stable, in-control, normally distributed process. Confirm with a control chart and a normality check before acting on these indices. Cpk here uses within (sample) sigma; long-term overall sigma gives Ppk instead.
Process capability: FAQ
What is Cpk?
Cpk is a process capability index that measures how well a process fits within its specification limits while accounting for how centred it is. It is the smaller of Cpu, based on the upper limit, and Cpl, based on the lower limit. A higher Cpk means fewer results are expected to fall out of specification.
What is the difference between Cp and Cpk?
Cp compares the width of the specification to the spread of the process (6 times sigma) and assumes the process is perfectly centred. Cpk also considers where the mean sits relative to the limits, so an off-centre process has a Cpk lower than its Cp. Cp is the best case; Cpk is what you actually get.
What Cpk is considered capable?
A common benchmark is a Cpk of at least 1.33, which corresponds to a comfortably capable process. Values from 1.00 to 1.33 are marginal and warrant improvement, while a Cpk below 1.00 means the process is not capable and is producing out-of-spec results. Your own quality standard or customer may set a stricter target.
What is the difference between Cpk and Ppk?
Cpk uses the within-subgroup or short-term (sample) sigma and describes the capability of a process that is in statistical control. Ppk uses the overall long-term sigma across all the data and describes actual performance over time. This tool computes Cpk from the sample standard deviation; if your sigma reflects long-term variation, read the same figure as Ppk.
Need your process validation to hold up in an audit?
We help pharmaceutical and device manufacturers set capability targets, build the statistical rationale, and document process validation so your data stands up in a PPB or ISO inspection.