What Cpk tells you
Cpk measures actual process capability by combining spread and centering. Even when Cp looks strong, Cpk can fall if the process average drifts toward the upper or lower specification limit.
Process Capability Guide
What Is Cpk?
Cpk is a process capability index that reflects both process variation and process centering. It shows how close the process mean is to the nearest specification limit, which makes it one of the most common metrics in manufacturing capability analysis.
Cpk formula and interpretation
Cpk is the smaller of CPU and CPL. CPU compares the process mean to the upper specification limit, while CPL compares the mean to the lower specification limit. The lower side becomes the effective Cpk because it reflects the closest risk point.
What is a good Cpk value
A Cpk of 1.33 is a common target for many production processes, although high-risk products or strict customer requirements may demand higher capability. The correct benchmark depends on context, not just a generic threshold.
Why Cpk matters in manufacturing
Because Cpk includes centering, it helps teams decide whether they should reduce variation, adjust the process target, or investigate a recent shift in setup, material, or measurement behavior.
How to read Cpk in real process reviews
When engineers review a low Cpk, they often ask two questions. First, is the process variation too wide? Second, is the average shifted toward one side? Looking at Cp together with Cpk helps separate those two cases. That distinction matters because variation reduction and centering correction are different improvement actions.
Why Cpk can change quickly
Cpk is sensitive to mean shifts, which means it can move quickly when the process target drifts. Tool wear, setup offsets, calibration issues, raw material changes, and measurement problems can all pull the process mean toward a specification limit. That is one reason Cpk is often monitored closely in ongoing production.
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 calculatorFrequently asked questions
What does Cpk measure?
Cpk measures actual process capability by considering both process spread and how close the process mean is to the nearest specification limit.
What is a good Cpk value?
A Cpk of 1.33 is a common target in manufacturing, although some high-risk processes need a higher value.
Why can Cpk be lower than Cp?
Cpk drops below Cp when the process is not centered, even if the underlying variation is relatively small.