The Cp formula
Cp = (USL - LSL) / (6 sigma). USL is the upper specification limit, LSL is the lower specification limit, and sigma represents the process standard deviation.
Process Capability Guide
Cp Formula
The Cp formula is one of the most common ways to estimate potential process capability. It compares the width of the specification limits with the natural spread of the process.
What the formula means
The numerator shows how much room the specification allows. The denominator shows how wide the process variation is. If the specification window is much wider than the process spread, Cp becomes larger.
How to interpret Cp results
If Cp is below 1.00, the process spread is wider than the specification width. If Cp is above 1.33, the process may have good potential capability, but only if the process is also centered.
Why Cp should be paired with Cpk
Cp alone does not tell you whether the process mean is close to the center of the tolerance. That is why teams usually review Cp and Cpk together instead of relying on only one index.
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