Step 1: Collect process data
Start with a set of measured values from your process. The data should come from the same characteristic and should reflect a meaningful production window rather than mixed conditions from unrelated setups.
Process Capability Guide
How to Calculate Cp and Cpk
To calculate Cp and Cpk, you need process data, an upper specification limit, a lower specification limit, the sample mean, and the process standard deviation. Once you have those values, the formulas are straightforward.
Step 2: Define USL and LSL
Set the upper specification limit and lower specification limit based on the engineering requirement for the feature you are measuring. Cp and Cpk are only meaningful when those limits are clear.
Step 3: Calculate mean and standard deviation
Calculate the process mean to understand centering, then calculate the sample standard deviation to estimate variation. These values are required for both Cp and Cpk formulas.
Step 4: Apply the formulas
Cp = (USL - LSL) / (6 sigma). CPU = (USL - mean) / (3 sigma). CPL = (mean - LSL) / (3 sigma). Cpk is the smaller of CPU and CPL. Once calculated, compare both Cp and Cpk to understand spread and centering together.
Process capability calculator
If you want to move from theory to practice, use the Cp / Cpk calculator to upload data, enter specification limits, and review capability metrics with a histogram and plain-language interpretation.
Try the calculator
Use the calculator on the main page to compare sample data, upload your own measurements, and see how Cp and Cpk change with different specification limits.
Open the calculator