To calculate the **Cpk index**, determine the process mean and standard deviation. Then, subtract the mean from both the upper specification limit (USL) and the lower specification limit (LSL). Divide these results by three times the standard deviation and choose the smaller value. This gives the Cpk index, which indicates process capability.

The **Cpk Index Calculator** measures the capability of a process to produce output within specified limits. The Cpk index measures how well a process is centered between its upper and lower specification limits. Where, a higher Cpk value indicates a more capable process.

For example, a **Cpk of 1.33** suggests that the process is capable of meeting the specification limits with minimal variation. This tool is widely used in Six Sigma and quality control processes. You can easily calculate the Cpk value using tools like **Cpk calculators in Excel** or even templates available online.

Along with that, it’s mandatory to understand how **Cpk** relates to sigma levels, as a **Cpk of 1.67** corresponds to approximately **4 sigma** capability.

**Formula:**

$Cpk = \min \left( \frac{USL - \text{mean}}{3\sigma}, \frac{\text{mean} - LSL}{3\sigma} \right)$

Variable | Description |
---|---|

$Cpk$ | Process capability index |

$USL$ | Upper specification limit |

$LSL$ | Lower specification limit |

$\sigma$ | Standard deviation |

Mean | Process mean |

**Solved Calculations:**

**Example 1**

Step | Calculations |
---|---|

USL and LSL | $USL = 10$, $LSL = 2$ |

Mean | $\text{mean} = 6$ |

Standard deviation | $\sigma = 1$ |

Apply the formula | $Cpk = \min \left( \frac{10 – 6}{3 \times 1}, \frac{6 – 2}{3 \times 1} \right)$ |

Final result | $Cpk = 1.33$ |

**Example 2**

Step | Calculations |
---|---|

USL and LSL | $USL = 12$ $LSL = 4$ |

Mean | $\text{mean} = 7$ |

Standard deviation | $\sigma = 1.5$ |

Apply the formula | $Cpk = \min \left( \frac{12 – 7}{3 \times 1.5}, \frac{7 – 4}{3 \times 1.5} \right)$ |

Final result | $Cpk = 0.67$ |

**What is a Cpk Index Calculator?**