Free Online Capability Index Calculator (Cp, Cpk, Cpm, Pp, Ppk)

Calculate short-term (Cp, Cpk, Cpm) and long-term (Pp, Ppk) capability indices. Includes normality tests, distribution plots, and I-Charts for robust analysis.

What is Process Capability (Cp, Cpk)?

Process Capability analysis is a fundamental tool in Statistical Process Control (SPC) used to measure the ability of a process to produce output within specification limits (LSL and USL). It helps determine if a process is capable of consistently meeting customer requirements.

This analysis uses key indices:

Cp (Process Capability): Measures the potential capability of the process. It compares the total specification width (USL - LSL) to the natural process variation (6-sigma), but it does not account for centering. It answers: "Is the process spread narrow enough?"
Cpk (Process Capability Index): Measures the actual capability of the process. It accounts for both spread and centering by comparing the process mean to the nearest specification limit. It answers: "Is the process spread narrow enough AND is it running near the target?"

Generally, a Cpk value of 1.33 or higher is considered capable for many industries, though this requirement can vary.

Brief History of Statistical Process Control (SPC)

The concept of process control is rooted in the work of Walter A. Shewhart at Bell Labs in the 1920s. He developed control charts to distinguish between "common cause" variation (the natural, inherent variability of a process) and "special cause" variation (external, unpredictable events). This was the birth of Statistical Process Control (SPC).

How to Use This Tool

Enter Process Data and Specifications: Add one or more datasets using the "Add Dataset" button. Fill in measurements for each dataset, along with the common Lower (LSL), Upper (USL), and Target values.
Calculate Results: Press the Calculate button. The tool will compute short-term indices (Cp, Cpk, Cpm) for each dataset, overall long-term indices (Pp, Ppk) combining all datasets, perform normality tests, and display process distribution plots and Individuals Control Charts.
Review Results: Analyze both the individual dataset results and the overall performance. Switch between datasets using the tabs to see detailed results for each one.
Add or Remove Datasets (Optional): Use the "Add Dataset" button to include more data or the "Remove" button next to each dataset to delete it.
Export Your Analysis: Click Export to Excel to download a spreadsheet containing all individual calculations and the overall performance summary.

Data Input

Overall Performance (Long-Term Analysis)

Mathematical Formulas

Sigma Within (Short-Term)

$$ \sigma_{\text{within}} = \frac{\overline{MR}}{d_2} $$

Where $d_2 = 1.128$ for $n=2$

Cp (Capability)

$$ C_p = \frac{USL - LSL}{6\sigma_{\text{within}}} $$

Cpk (Centering)

$$ C_{pk} = \min\left( \frac{USL - \mu}{3\sigma_{\text{within}}}, \frac{\mu - LSL}{3\sigma_{\text{within}}} \right) $$

Cpm (Targeting)

$$ C_{pm} = \frac{USL - LSL}{6\sqrt{\sigma_{\text{within}}^2 + (\mu - T)^2}} $$

Pp (Performance)

$$ P_p = \frac{USL - LSL}{6\sigma_{\text{overall}}} $$

Ppk (Centering)

$$ P_{pk} = \min\left( \frac{USL - \mu}{3\sigma_{\text{overall}}}, \frac{\mu - LSL}{3\sigma_{\text{overall}}} \right) $$

Capability Legend

Cp, Pp: Process Capability/Performance - measures potential capability if process is centered
Cpk, Ppk: Process Capability/Performance Index - accounts for process centering
Cpm: Process Capability Index considering target value
LSL, USL: Lower and Upper Specification Limits
T: Target value
μ: Process mean
σ_within: Short-term standard deviation (within-subgroup variation)
σ_overall: Long-term standard deviation (overall variation)