Free Cp Cpk Calculator — process capability with PPM estimate

Free Cp & Cpk Calculator

Enter your spec limits and process statistics — get Cp, Cpk, and an estimated defect rate instantly, in your browser.

Calculator

Upper spec limit (USL)

Lower spec limit (LSL)

Process mean

Standard deviation (σ)

Cp (potential)

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Cpk (actual)

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Est. defects (PPM)

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How Cp and Cpk are calculated

Cp = (USL − LSL) ÷ 6σ Cpk = min(USL − mean, mean − LSL) ÷ 3σ

Cp asks "could the process fit the spec if perfectly centered?" Cpk asks "does it fit where it's actually running?" The gap between them is your centering problem. With the defaults: Cp 1.39, Cpk 1.33 — the classic minimum for an existing process (1.67 is the usual target for new launches and safety characteristics).

A note on σ

Enter within-subgroup sigma (from R̄/d₂) and you get true Cp/Cpk; enter overall sigma and the result is technically Pp/Ppk. The distinction matters for PPAP — the full workbook computes both from your raw measurements and says which is which.

This calculator does the single calculation. The workbook does the whole study.

The full workbook takes 300 raw measurements, computes within and overall sigma properly (Cp/Cpk and Pp/Ppk), estimates PPM, and draws the histogram your PPAP reviewer expects.

See the Process Capability Calculator (Cp·Cpk·Pp·Ppk) workbook — $24 →