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Cp, Cpk, Pp and Ppk Calculator

This process capability calculator compares specification width with short-term and overall process variation. Paste measurements, enter LSL and USL, then review centring and spread before making a capability decision.

Run the calculation

Your data stays in this browser. No measurements are uploaded.

  1. 1 Enter data
  2. 2 Review results
  3. 3 Take action

Study inputs

Readings remain in sequence because within sigma uses successive moving ranges.

Ready for your data

Complete the inputs or load the worked example. Results will include the primary decision, interpretation and supporting metrics.

Worked example

For measurements around 10.000 mm with LSL 9.950 mm and USL 10.050 mm, the tool estimates spread and centring separately. Load the example to reproduce the calculation and compare Cpk with Ppk.

Load the example in the calculator

Formula and methodology

  1. The mean and overall sample standard deviation are calculated from all readings.
  2. Within-process sigma is estimated from the average moving range divided by d₂ = 1.128 for successive pairs.
  3. Cp and Cpk use within sigma; Pp and Ppk use the overall sample standard deviation.
Cp = (USL − LSL) / (6σwithin)Cpk = min[(USL − mean) / (3σwithin), (mean − LSL) / (3σwithin)]Pp and Ppk use σoverall in the same equations.

How to interpret the result

  • Cp below the required threshold indicates excessive short-term spread.
  • Cpk lower than Cp indicates that the mean is closer to one specification limit.
  • Capability indices should be interpreted only after process stability and distribution assumptions have been assessed.

Common errors and limitations

  • Using specification limits as control limits.
  • Treating a small or unstable dataset as evidence of long-term capability.
  • Comparing indices calculated with different sigma estimators without documenting the method.

Turn the calculation into an engineering decision

Share the drawing, process conditions and result context. AccuTouch can review the measurement, fixture, SPC or automation starting point.

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Frequently asked questions

What is the difference between Cp and Cpk?

Cp describes potential capability from spread alone. Cpk also reflects how close the process mean is to the nearest specification limit.

Why can Ppk be lower than Cpk?

Ppk uses overall variation, which can include drift and between-subgroup changes that a within-process estimate excludes.

How many readings are enough?

There is no universal minimum. More data across representative time and operating conditions improves confidence; assess stability before capability.

Technical references

Use the current controlled standard or your organisation’s approved method where it differs from this screening tool.

Method content reviewed 13 August 2026. Independent technical approval is still required before production publication or use in a controlled quality procedure.