Process Capability Cp and Cpk Calculator
Compare an entered process mean and standard deviation with two specification limits and estimate normal-model tail proportions.
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Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Compare the six-sigma process spread with specification width. Calculate the standardized distance to each specification and use the smaller for Cpk.
Cp=(USL−LSL)/(6σ); Cpk=min[(USL−mean)/(3σ),(mean−LSL)/(3σ)]
Worked example
Mean 10, sigma 1 and specifications 7–13 give Cp=Cpk=1. A normal model places about 0.26998% outside these ±3 sigma limits.
Assumptions and input details
Inputs share one measurement unit, sigma is positive and upper specification exceeds lower. These are specification limits, not control-chart limits. Capability interpretation requires a stable process and suitable variation estimate; normal tails assume a normal distribution. This calculator does not verify these assumptions from raw observations. No universal acceptance grade, confidence interval or shifted-sigma convention is imposed. Extremely small normal tails may round to zero.
References: NIST: process capability
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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