Quality & conditional normal-model indices

Process Capability Calculator

Calculate Cp and Cpk only when your process study affirms stability and a normal-model basis.

183 / PROCESS CAPABILITY

Your numbers.
A clearer decision.

Free to use. No signup.
Inputs stay in this browser.

INPUT GROUP 01

Entered assumptions and matching units

Lower specification limitUpper specification limitMean from relevant process studyPositive process standard deviationSample count supporting the estimatesStatistical stability established independentlyNormal-model interpretation justified independently

Check the assumptions before acting on a result.

Read the method and example

YOUR CALCULATION

Calculate conditional normal-model capability indices.

Sample size supporting your supplied estimates

100

Cp if the declared assumptions hold

1.667

Cpk if the declared assumptions hold

1.667

Upper-specification distance in standard deviations

Lower-specification distance in standard deviations

Keep the calculation

Saving is optional and stays on this device. Unsaved edits are not kept when you leave. CSV includes every input and the current results.

Illustrative inputs loaded. Replace them with your own assumptions.

How the process capability calculator works

Cp divides specification width by six SD. Cpk is the smaller distance from the mean to either specification divided by three SD. The upper specification must exceed the lower and SD must be positive; negative Cpk remains visible for a mean outside a limit.

Stability and normal-model selections are your assertions from a suitable study, not tests performed here. If either is unestablished, Cp and Cpk are withheld. Sample size is shown for context without asserting adequacy.

No universal acceptable threshold, confidence bound, defect forecast, product acceptance or process certification is supplied. Estimated SD must represent the intended process study; an overall and a within-subgroup estimate are not automatically interchangeable.

Worked example

With limits 90 and 110, mean 100 and SD 2, Cp and Cpk are both about 1.666667 under affirmed assumptions. Moving the mean to 108 leaves Cp unchanged but gives Cpk = 2/6, about 0.333333.

Sources and assumptions

The source material explains the underlying calculation or pricing structure. Entered prices, costs, schedules and scenario choices are your assumptions; this tool does not obtain supplier quotes or predict actual results.

NIST: process capability indices and assumptions

Calculation methods, testing and sources · Practical calculation guides