Cp, Cpk, Pp and Ppk Calculator

Capability (Cp, Cpk) and Performance (Pp, Ppk) Calculator

Measured Data
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Results and Histogram
Cp
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Cpk
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Pp
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Process Capability Report

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Author: Pavel Ryvola | Updated:

Cp, Cpk, Pp and Ppk Calculator

A simple, instant, and visual process capability calculator without installing unnecessary software. Paste your data and find out if your process is centered and within tolerances.

Cp vs Cpk: What is the difference?

Cp Capability (Potential)

Focuses on the short-term potential of the machine without external influences. It shows if the data spread can physically fit into the tolerance, but does not consider centering of the process.

Cpk Capability (Actual)

It is stricter. It considers the centering of the process – monitoring how dangerously close you are to the worst tolerance limit (LSL or USL). Cpk always takes the worse value.

Cpk Formula

The basic principle of the calculation lies in comparing the required tolerance (what the customer wants) and the actual process variation (what our machine can actually do). To do this, we need to know the mean of the data and its standard deviation (σ).

Calculating Cp (Process Potential)

Formula: Cp= USL-LSL 6×σ

Indicates whether the data variation physically fits within the tolerance band. USL is the Upper Specification Limit, LSL is the Lower Specification Limit, and σ (sigma) is the standard deviation. Cp does not account for process centering.

Calculating Cpk (Process Capability)

Formula: Cpk=min(Cpkupper,Cpklower)
Cpkupper= USL-x¯ 3×σ Cpklower= x¯-LSL 3×σ

Where x̄ is the mean. Cpk calculates the distance of the mean from both specification limits. It takes the worse (smaller) value as the result, because this represents a higher risk of producing a defect (nonconformity).

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.

Learning Center: Cpk Guides

Cp/Cpk vs. Pp/Ppk

Read about the difference in statistics and real-world operations.

What does the Cpk value mean?

Learn how to convert the Cpk value to PPM (defective parts).

Top Defect

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).

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.

Generally, a Cpk value ≥ 1.33 is considered a safely capable process. In the automotive industry, critical characteristics often require excellent capability with a Cpk ≥ 1.67.