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Dimensional Process Capability (Cp / Cpk)

Use drawing limits, process mean, and within-subgroup variation to review Cp, Cpk, and dimensional margin.

Example starting values are prefilled to demonstrate the calculation. Replace them with your measured, quoted, or job-specific inputs before relying on a result.

Inputs

Drawing limits and process data

Enter the lower drawing limit for the same measured characteristic and datum/measurement method.

Enter the upper drawing limit for the same measured characteristic. It must be greater than LSL.

Mean measured from the defined stable process and representative production sample.

Range: at least 0.0 s

Use a positive within-subgroup sigma estimate from rational subgroups, in the same dimension unit. Do not substitute total long-term standard deviation without changing interpretation.

Defaults to inches for US drawings; all four dimensional inputs use this unit.

Result

Potential capability Cp
3.333
Centered capability Cpk
3.333
Mean distance above LSL (in)
0.05000
Mean distance above LSL (mm)
1.2700
Specification width (in)
0.10000
Specification width (mm)
2.5400
Mean distance below USL (in)
0.05000
Mean distance below USL (mm)
1.2700
Mean within specifications
Statistical assumptions and limitation
Cp measures specification width relative to six within-subgroup standard deviations; Cpk also accounts for the process mean’s distance to the nearer limit. These indices assume a stable process and a defensible within-subgroup sigma. They are not a control-chart/stability test, normality test, measurement-system study, sample-size confidence interval, or pass/fail decision; apply customer-specific criteria separately.

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