Process Capability (Cpk) Calculator
Calculate the process capability index (Cpk) and analyze your process performance.
What is the Process Capability Index (Cpk)?
The Cpk, or Process Capability Index, is a statistical metric used in quality control to measure a process's ability to produce an output within the specification limits defined by the customer or design. In essence, Cpk tells you how "capable" your process is.
A high Cpk value indicates that the process is stable, predictable, and well-centered within the specification limits, resulting in fewer defects. Conversely, a low Cpk suggests that the process has high variability or is not centered, increasing the likelihood of producing out-of-spec parts.
Brief History of Cpk
The concept of process capability is rooted in the work of Walter A. Shewhart at Bell Labs in the 1920s, who is considered the father of Statistical Process Control (SPC). Shewhart developed control charts to distinguish between "common cause" variation (inherent to the process) and "special cause" variation (external events).
Building on these foundations, indices like Cp and Cpk emerged in subsequent decades, especially with the rise of total quality movements in Japan and the Six Sigma methodology popularized by Motorola in the 1980s. These metrics became standard tools in the manufacturing industry to quantify quality and guide continuous improvement efforts.
How to Use This Tool
- Enter Measurements: In the first field, type your process data separated by commas, or click the "Fill Example" button to load sample data.
- Define Specifications: You have two options:
- By Limits: Directly enter the Lower Specification Limit (LSL) and Upper Specification Limit (USL).
- By Nominal & Tolerance: Enter the target value (Nominal) and the allowed tolerances. The tool will calculate the LSL and USL for you.
- Calculate: Press the "Calculate Cpk" button.
- Analyze Results:
- Numerical Results: Review the calculated values for Mean, Standard Deviation, Cpk, and the expected percentage of defects. Generally, a Cpk ≥ 1.33 (shown in green) is desired for a capable process. A value between 1.0 and 1.33 (yellow) is marginal, and less than 1.0 (red) indicates a non-capable process.
- Capability Chart: Visualize the distribution of your data (bell curve) in relation to the specification limits (red dashed lines) and the process mean (green line). The chart also shows standard deviation markers (σ).
- Add and Export: You can add multiple calculations to a list and then export all results to an Excel file for your reports.