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Process Capability (Cp Cpk) Calculator

Work out process capability (cp cpk) instantly with clear inputs, formula shown and shareable results.

About Process Capability (Cp Cpk) Calculator

Cp compares the specification width with six standard deviations of process spread and assumes the process is perfectly centred. Cpk penalises off-centring by taking the worse of the two one-sided indices, so the gap between Cp and Cpk is exactly the capability you are losing to a mean that has drifted.

Formulas

Cp

Cp = (USL - LSL) / (6 x sigma)

Cpk

Cpk = min[(USL - mean)/(3 sigma), (mean - LSL)/(3 sigma)]

Worked example

With the default values below, Process Capability (Cp Cpk) Calculator returns Cpk (actual capability) of 1.077. Change any input above and every figure here is recalculated in your browser.

Inputs used

Upper specification limit
10.5
Lower specification limit
9.5
Process mean
10.08
Process standard deviation
0.13

Results

Cpk (actual capability)
1.077
Cp (potential capability)
1.282
Capability lost to off-centring
0.205
Verdict
Marginal

Step by step

  1. 1Values used
  2. 2Cp
  3. 3Cpk
  4. 4Cpk (actual capability)= 1.077
  5. 5Cp (potential capability)= 1.282
  6. 6Capability lost to off-centring= 0.205
  7. 7Verdict= Marginal

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • This is an estimate for general information and not professional financial, tax or legal advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What Cpk is acceptable?

1.33 is the usual minimum for an established process and 1.67 for safety-critical or new-product characteristics. Below 1.0 the process is producing out-of-specification parts.

Why is Cpk always at or below Cp?

Because Cpk uses the distance to the nearer specification limit. They are equal only when the mean sits exactly at the midpoint.

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Process Capability (Cp Cpk) Calculator | Calcrivo