Process Capability Index (Cp, Cpk) Calculator

About this calculator

This calculator computes the two core Six Sigma process capability indices from a process's spec limits, mean and standard deviation. Cp = (USL − LSL) ⁄ 6σ compares the width of the tolerance band to the natural spread of the process (six standard deviations), assuming the process is perfectly centred — it answers “could this process fit the spec if it were centred?”

Cpk = min[(USL − μ) ⁄ 3σ, (μ − LSL) ⁄ 3σ] is the more realistic sibling: it accounts for where the process mean μ actually sits relative to the limits, taking the tighter of the two one-sided distances. Cpk is always ≤ Cp, and the gap between them tells a quality engineer whether a process needs to be re-centred, tightened, or both — a Cpk of 1.33 or higher is a common industry threshold for a “capable” process.