Cpk Calculator

Process capability calculator with raw measurement input, mean and standard deviation mode, target Cpk planning, Cp and Cpk interpretation, and a histogram with distribution plot.

1. Paste Measurement Data

Use this mode if you have actual sample measurements, such as 30, 50, or 100 values from inspection results.

Paste values separated by commas, spaces, or new lines.

2. Basic Statistics

Paste measurement data to see results.

3. Specification Limits

Enter your upper and lower specification limits.

Formulas
Cp = (USL - LSL) / 6σ
CPU = (USL - Mean) / 3σ
CPL = (Mean - LSL) / 3σ
Cpk = min(CPU, CPL)

4. Cp / Cpk Results

Enter data first.

5. Distribution Plot

Paste data to see the histogram and curve.

Cp, Cpk, CPU, and CPL Explained

Learn what each process capability metric means, how to interpret Cpk values like 1.00 or 1.33, and why Cp can differ from Cpk.

What Cp means
Cp compares your tolerance width with your process spread. A higher Cp means your process variation is small enough to fit inside the specification limits, assuming the process is centered.
What Cpk means
Cpk measures actual capability after considering both variation and centering. If the process drifts toward one limit, Cpk will drop even when Cp still looks good.
What CPU and CPL mean
CPU shows capability toward the upper limit. CPL shows capability toward the lower limit. Cpk uses the smaller of the two, because the weaker side controls the real risk.
Cp = 1.00 example
Cp of 1.00 means the process spread is about the same width as the tolerance band. In simple terms, the process just fits. There is little room for drift or shift.

Difference between Cp and Cpk

Cp only looks at spread. Cpk looks at spread and centering. That is why Cp can be good while Cpk is poor if the process average is drifting toward USL or LSL.

Common Cpk interpretation

  • Cpk below 1.00: process is usually not capable.
  • Cpk around 1.00 to 1.33: process is marginal.
  • Cpk 1.33 and above: commonly accepted in many manufacturing applications.
  • Cpk 1.67 and above: stronger capability with more margin.

Why sample size matters

Cp and Cpk estimates become more useful when they are based on enough data. Small samples can give unstable results. As a practical starting point, many users begin with at least 30 measurements.

Cp & Cpk Calculator (Process Capability)

Use this free Cp and Cpk calculator to evaluate process capability from raw measurements or from mean and standard deviation. Get Cp, Cpk, CPU, CPL, and interpretation instantly.

What is Cp?

Cp measures potential capability assuming the process is centered: Cp = (USL − LSL) / (6σ).

What is Cpk?

Cpk measures actual capability considering centering: Cpk = min((USL − μ)/(3σ), (μ − LSL)/(3σ)).

Interpretation

  • Cpk < 1.00 → Not capable
  • Cpk ≈ 1.33 → Industry minimum
  • Cpk ≥ 1.67 → Good
  • Cpk ≥ 2.00 → Excellent
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