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Process Capability (Cp/Cpk) Calculator

Calculate process capability indices Cp, Cpk, and Cpm from process statistics and specification limits. Determine if your process meets quality requirements.

Inputs

Results

Cp (Potential Capability)

1.67

Cpk (Actual Capability)

1.67

Assessment

Good capability

Cpm (Taguchi Index)1.67
Cpu (Upper)1.67
Cpl (Lower)1.67
Sigma Level5
Est. Defects per Million1
Est. Yield100%
How to Use This Calculator
  1. Enter Process Mean (X̄), Process Std Dev (σ), and Upper Spec Limit (USL).
  2. Set Lower Spec Limit (LSL).
  3. Review Cp (Potential Capability), Cpk (Actual Capability), and Assessment.
  4. Use Cpm (Taguchi Index) and Cpu (Upper) to inform your decision.
  5. Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Upper Spec Limit (USL)

Upper Spec Limit (USL)Cp (Potential Capability)Cpk (Actual Capability)Assessment
-800,000-133,340.83-266,683.33Not capable — process needs improvement
-300,000-50,007.5-100,016.67Not capable — process needs improvement
300,00049,992.51.67Good capability
800,000133,325.831.67Good capability

What each input means

Process Mean (X̄)
The average output of your process. Centering affects Cpk.
Process Std Dev (σ)
Process standard deviation — measures the natural variation. Use within-subgroup sigma for short-term capability.
Upper Spec Limit (USL)
The maximum acceptable value defined by customer or engineering requirements.
Lower Spec Limit (LSL)
The minimum acceptable value defined by customer or engineering requirements.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Process Mean (X̄) = 50, Process Std Dev (σ) = 1, Upper Spec Limit (USL) = 55, Lower Spec Limit (LSL) = 45 = 4 input(s) provided
  2. Calculate Cp
    Cp
    1.6667 = 1.6667
  3. Calculate Cpk
    Cpk
    1.6667 = 1.6667
  4. Calculate Assessment
    Good capability = Good capability
  5. Calculate Cpm
    Cpm
    1.6667 = 1.6667
  6. Calculate Cpu
    Cpu
    1.6667 = 1.6667

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