Process Capability Index (Cp)

What is this formula?

The Process Capability Index (Cp) measures the potential capability of a manufacturing process by comparing the specification width with the natural variability of the process.

It indicates whether a process is theoretically capable of producing within specification limits assuming the process is perfectly centered.

When to use it

Use this formula when:

- Evaluating manufacturing quality.

- Assessing process capability.

- Comparing production processes.

- Supporting Six Sigma initiatives.

- Monitoring quality improvement programs.

Example

A machining process has:

USL = 50.5 mm

LSL = 49.5 mm

σ = 0.12 mm

Formula:

Cp = (USL - LSL)/(6σ)

Substitution:

Cp = (50.5 - 49.5)/(6×0.12)

Cp = 1/0.72

Cp = 1.39

Result:

The process capability index is 1.39.

Applications

- Statistical Process Control

- Six Sigma

- Manufacturing quality

- Process improvement

- Industrial quality engineering

Note

Cp measures potential capability only and assumes the process mean is centered between specification limits. A process can have a high Cp value yet still produce defects if the process average is shifted away from the target value. For this reason, Cp is often evaluated together with Cpk.