Everyday process capability index problems come down to one relationship between lower specification limit, mean, standard deviation, target and upper specification limit. This calculator turns those numbers into process capability around a target (cₚₖₘ), potential process capability (cₚ) and process capability index (cₚₖ), with the working shown so you can check the result rather than simply trust it.
What Is the Process Capability Index Calculator?
The process capability index calculator turns a set of measurements into process capability around a target (cₚₖₘ), potential process capability (cₚ) and process capability index (cₚₖ), with the uncertainty made explicit rather than left implied.
Statistics is easy to quote badly. A figure without its spread tells the reader far less than they assume it does, and this calculator puts both together.
How the Process Capability Index Is Calculated
The calculation uses the standard relationship for this kind of estimate, applied to the figures you supply. The intermediate steps are exposed so you can see how much of the answer comes from each one.
Sample size matters more than most people expect. Doubling the number of observations improves the precision by a factor of about one and a half, not two.
What the Result Does and Does Not Tell You
A result describes the data you gave it. It does not tell you whether the sample was representative, whether the method was appropriate, or whether there is a confound in the way the figures were collected.
Report the result together with its uncertainty. A number on its own is much easier to misread than a number with a range attached.
A Worked Example
Working through an example, with lower specification limit set to 10, mean set to 5, standard deviation set to 2, the calculator returns potential process capability (cₚ) of 0, and process capability index (cₚₖ) of -0.833, and taguchi capability index (cₚₘ) of 0. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out process capability around a target (cₚₖₘ)?
Enter lower specification limit, mean, standard deviation, target and upper specification limit. The calculator then works through the relationship and reports process capability around a target (cₚₖₘ), potential process capability (cₚ) and process capability index (cₚₖ).
What does the process capability index actually calculate?
It takes lower specification limit, mean, standard deviation, target and upper specification limit and converts them into process capability around a target (cₚₖₘ), potential process capability (cₚ) and process capability index (cₚₖ) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter lower specification limit?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.