Cp is a process capability index used in statistical process control and quality engineering. It commonly refers to the ratio between the allowed specification width and the natural spread of a process, typically estimated as six standard deviations.
In practical terms, Cp indicates the potential capability of a process to fit within upper and lower specification limits if the process is stable and centered between those limits. A higher Cp value means the process variation is small relative to the tolerance range.
Cp does not show whether the process average is actually centered on target. Because of that, it does not by itself describe the actual defect risk when the process mean is shifted. For that reason, Cp is often reviewed alongside Cpk, which accounts for centering.
What Cp includes and excludes
- Includes: comparison of process variation to specification limits.
- Assumes: a reasonably stable process and a meaningful estimate of variation.
- Excludes: process centering, special-cause instability, and broader system issues such as measurement error unless those are addressed separately.
How it appears in manufacturing
Cp is commonly used for critical dimensions, fill volumes, torque values, temperature-controlled steps, and other measurable characteristics in production and quality workflows. It may appear in SPC software, MES-connected quality records, capability studies, control plans, supplier quality reviews, and continuous improvement reporting.
For example, a machining process may show a high Cp for a diameter tolerance, meaning the observed variation is narrow compared with the specification band. If the process mean drifts toward one limit, however, actual performance may still be unacceptable even though Cp remains high.
Common confusion
Cp vs. Cpk: Cp measures potential capability based on spread only. Cpk measures capability while also considering how centered the process is within the specification limits.
Cp vs. Pp: Cp is commonly associated with short-term or within-process variation in a stable process. Pp commonly uses overall performance variation across a broader time window.
Cp vs. control limits: Cp uses specification limits set by design or customer requirements, not control limits calculated from process behavior.