NTB SpecPro

Cp / Cpk process capability calculator

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Free tool · Quality control

Cp / Cpk process capability calculator

Paste your measurements or enter the mean and standard deviation, give the tolerance limits (USL/LSL); Cp, Cpk, Pp, Ppk and a simple histogram appear instantly. Everything runs in your browser; nothing is sent.

Also available in: Türkçe · العربية

Number of measurements n–
Mean–
σ (moving range, within)–
s (overall)–
Cp–
Cpk–
Pp–
Ppk–
Common practice–

Histogram

Cp = (USL − LSL) / (6σ) · Cpk = min[(USL − x̄)/(3σ), (x̄ − LSL)/(3σ)] · σ = MR̄ / 1.128 (moving range of consecutive measurements) · Pp, Ppk use the same formulas with the overall s · With one limit only that side is calculated

Cp/Cpk describe short-term (within) variation, Pp/Ppk long-term (overall) variation; the moving-range method is meaningful when measurements are pasted in time order. When mean and s are entered directly, Cp = Pp. The formulas assume a normal distribution; if the data is skewed or the measurement system is at fault, solve that first.

For information only; the customer specification and the control plan govern. The final assessment rests with the responsible quality engineer.

Let the app keep your measurement records and SPC trackingThe Quality control module offers drawing ballooning, measurement records, SPC tracking and AS9102 FAI reports.
Try the Quality control module

Curious?

What is the difference between Cp and Cpk?Cp is the ratio of the tolerance to the process spread; it ignores whether the process is centred. Cpk uses the distance of the mean to the nearest limit; if the process shifts, Cpk falls while Cp stays. The gap between the two shows a centring problem.
Why do Pp/Ppk differ from Cp/Cpk?Cp/Cpk use short-term variation (from the moving range between consecutive measurements), Pp/Ppk the standard deviation of all data. If the process drifts over time the overall spread grows and Pp/Ppk come out lower; a large gap is a sign of an unstable process.
How many measurements are needed?The common recommendation is at least 30, preferably 50–100; with little data the uncertainty of Cpk is large. For first articles or small lots the result is treated as approximate and noted in the control plan.
Where do I get the measurements and how do I track them?The Quality control module of the app balloons the drawing, records every dimension, continues SPC tracking under the same part number and produces the AS9102 FAI report; Cp/Cpk is not typed into a spreadsheet by hand.

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