Process Capability Calculator: Cp and Cpk Made Simple

By Robert Low, Director, Lead Auditor & BRCGS Approved Trainer

Cp and Cpk tell you whether your process can reliably hit its limits, not whether it hit them yesterday. My free process capability calculator works them out from your mean, standard deviation and specification limits.

What the Process Capability Does

Cp compares the width of your specification with the natural spread of your process. Cpk does the same but also punishes you if the process is off centre. A process can have a great Cp and a poor Cpk because it is running close to one limit.

A common target for a capable process is a Cpk of 1.33 or better. Below 1.0, you will produce out of tolerance product even when everything is running normally.

How to Use It

1. Open Process Capability on the free resources page.
2. Enter the upper and lower specification limits.
3. Enter the process mean and standard deviation from your data.
4. Read Cp and Cpk and decide whether the process needs centring, tightening or both.

Where It Helps Beyond Certification

Capability data is excellent evidence in a customer conversation. If a customer asks whether you can hold a tighter tolerance, a Cpk figure from real data beats a promise. It also tells you when to invest in a better filler or a different tool, and when the problem is just setup.

It supports the legal side too. If your fill process is capable against the tolerable negative error, you spend far less time worrying about packaged goods checks.

Where Sites Get Caught Out

Capability only means something if the process is stable. Plot your data first. If it is drifting or has special causes in it, the Cpk is not telling you the truth.

Related free tools: Gage R&R (Quick).