What is process capability (Cp, Cpk)?

Process capability indices (Cp, Cpk) in Statistical Process Control (SPC) indicate how well a stable manufacturing process can meet the specified tolerance limits (LSL and USL).

What is the difference between Cpk and Ppk?

Cp and Cpk focus on short-term variability within subgroups. In contrast, Pp and Ppk (process performance) account for the overall long-term variation, including all machine mean shifts.

Cpk Calculator: Process Capability (Cp, Cpk)

Report Title (for PDF header)

LSL

Target

USL

Measured Data

Paste measured values from an Excel column (one value per line).

Results and Histogram

Cpk

Ppk

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Cpk Calculator: Process Capability (SPC) Made Simple

Statistical Process Control (SPC) sometimes sounds like rocket science, but in practice, it's about one thing: being certain your production churns out good parts consistently and easily fits within customer tolerances. Our online Cpk calculator is here to save you time. No need to install software like Minitab or create complex Excel spreadsheets. Just paste your data and instantly get an accurate process capability calculation (Cp, Cpk) and long-term performance (Pp, Ppk).

Cp/Cpk vs. Pp/Ppk: What Is the Actual Difference?

Process Capability (Cp, Cpk) – Your Ideal Scenario: Focuses on the short-term potential of the machine without external influences. While Cp alone only tells you if the data spread physically fits within the tolerance, Cpk is stricter. It accounts for process centering – keeping an eye on how dangerously close you are to the worse tolerance limit (LSL or USL).
Process Performance (Pp, Ppk) – The Hard Reality: Evaluates long-term performance and uses all fluctuations over the monitored period for the calculation. This reflects shift changes, tool wear, a new material batch, or simply the fact that it got colder in the production hall in the morning.

What Is a "Good" Cpk Value? (And When to Watch Out)

The golden rule is: If Cpk is significantly lower than Cp, the process is not well-centered (you're shooting accurately, but off-target). But what numbers does your customer actually want to see?

Cpk < 1.00: Trouble brewing. The process is not capable, and you are most likely producing scrap.
Cpk = 1.00 až 1.33: Living on the edge. You are producing good parts, but a minor machine hiccup will put you out of bounds.

Cpk ≥ 1.33: Peace of mind. The process is safely capable and meets standard industry requirements.
Cpk ≥ 1.67: Excellent machine capability. The automotive industry will demand this number for critical characteristics.

How to Use the Online Cpk Calculator?

It's extremely straightforward. Enter the lower specification limit (LSL) and upper limit (USL) from the drawing into the left panel. Then paste your measured data (we recommend 30 to 50 samples) – directly from your Excel column, for example. The calculator automatically creates a professional histogram overlaid with a smoothed normal distribution curve in a design quality engineers are used to. You can then download the resulting SPC report as a PDF with a single click.

Process Capability Calculator