DATUMKEEP
Free tool

Cp and Cpk, with the uncertainty left in

Paste your measurements, enter your spec limits, and get Cp and Cpk with 95% confidence intervals. If your sample is too small to support a conclusion, this tool says so instead of printing a confident number.

Nothing is uploadedNo sign-upInterval, not just a point estimate
Your data

Commas, spaces, tabs, semicolons or one per line. Decimal commas are understood; anything that is not a number is ignored.

At least one limit is required. With only one you get a one-sided Cpk (Cpu or Cpl); Cp needs both.
Your data never leaves the browser. Every number on this page is computed locally in this tab. There is no upload, no account and no server call — close the tab and it is gone.
Result
Waiting for data

Paste your measurements and at least one spec limit. Everything is computed here, in this tab — nothing is uploaded.

Distribution · your readings against the spec
Paste measurements to see the distribution
Bars are your readings; the dashed curve is the normal distribution the indices assume. If the bars and the curve disagree, the indices are describing a process that is not there.

How to read the numbers

Cp is potential, Cpk is reality

Cp compares your spread to your tolerance and ignores where the process sits. Cpk penalises being off-centre. A large gap between the two means a centring problem, not a variation problem — and centring is usually the cheaper fix.

The interval is the answer, not the point estimate

A Cpk of 1.35 from 20 parts and a Cpk of 1.35 from 300 parts are not the same claim. The confidence interval is what separates them. Read the lower bound: that is the capability your data actually supports.

Small samples lie confidently

Below 15 readings the interval is so wide that almost any conclusion fits inside it. This tool refuses to dress that up — it leads with the warning, then shows the arithmetic anyway so you can see why.

Capability assumes a stable process

Capability describes a process that is in statistical control. If the process drifted while the parts were made, a capability index summarises a distribution that never existed. Check the control chart first, always.

This is a snapshot. DatumKeep does this continuously

A paste-in calculator tells you about one sample on one day. DatumKeep runs capability with intervals on every characteristic, watches the control charts for the instability that invalidates them, and turns a violation into a root-cause analysis with the evidence already attached.