🏭 Process Capability Index Calculator
Enter the upper and lower specification limits, the process mean, and the standard deviation to calculate the Cp and Cpk process capability indices used in quality control.
What this calculator does
Process capability indices measure how well a manufacturing or business process output fits within its specification limits, assuming the output is approximately normally distributed. Cp and Cpk are the two most common indices used in quality control and Six Sigma programs.
How the calculation works
Cp compares the width of the specification range to the natural spread of the process: Cp = (USL – LSL) / (6 x standard deviation). This measures potential capability but ignores whether the process is centered. Cpk accounts for centering by taking the smaller of two one-sided indices: CPU = (USL – mean) / (3 x SD) and CPL = (mean – LSL) / (3 x SD); Cpk = the minimum of CPU and CPL.
As a rule of thumb, a Cpk of 1.33 or higher is often considered capable, while a Cpk below 1.0 suggests the process regularly produces output outside spec. The calculator also estimates the percentage of output expected to fall within the specification limits using the normal distribution. This tool is for general quality-control education and planning; real capability studies should also check for normality and process stability.