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Cpk Calculator
Paste your process data and spec limits — get Cp, Cpk, Pp, Ppk, your sigma level and DPMO instantly. Runs in your browser; nothing is uploaded.
Runs in your browser. Nothing is uploaded. Same engine as SigmaForge DataForge™.
Enter your data and spec limits, then hit Calculate to see Cp, Cpk, Pp, Ppk, your sigma level, and DPMO.
How to calculate Cpk
Process capability compares the voice of the process (its natural variation) to the voice of the customer (the specification limits). This calculator computes it the same way a Six Sigma Green Belt would: it estimates the process mean and both short-term (within) and long-term (overall) standard deviation from your data, then derives Cp, Cpk, Pp, and Ppk.
Cpk value guide
| Cpk | Verdict | Approx. DPMO |
|---|---|---|
| < 1.00 | Not capable | > 2,700 |
| 1.00 – 1.33 | Marginal | 63 – 2,700 |
| 1.33 – 1.67 | Capable | 0.6 – 63 |
| ≥ 1.67 | Excellent | < 0.6 |
FAQ
What is Cpk?
Cpk (process capability index) measures how well a process fits within its specification limits, accounting for how off-center it is. It is the smaller of (USL − mean)/(3σ) and (mean − LSL)/(3σ). Higher is better.
What is a good Cpk value?
A Cpk of 1.33 is the common industry threshold for a capable process (~63 DPMO). 1.00 means the process barely fits its spec; 1.67+ is often required for critical or safety characteristics.
What is the difference between Cpk and Ppk?
Cpk uses within-subgroup (short-term) sigma, estimated here from the average moving range (R̄/d₂). Ppk uses overall (long-term) sigma, the ordinary sample standard deviation. Cp/Cpk describe potential; Pp/Ppk describe actual performance.
How many data points do I need?
You need at least 2 to compute anything, but for a trustworthy capability estimate use 30+ measurements from a stable, in-control process. Capability on an unstable process is not meaningful.
Go beyond the number
SigmaForge's DataForge™ adds the capability histogram, a plain-English interpretation, and a board-ready branded PDF — plus 23 more analyses. Start free with White Belt certification.
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