chaaam.

Manufacturing quality

SPC, Cp, Cpk and Ppk Calculator

Import or paste numerical inspection readings to analyze center, spread and process capability against entered specification limits. The chart identifies individual points beyond the estimated three-sigma I-chart limits.

Use free calculator Open full Engineering Workspace ↗

Browser-based tool No account Input files stay local

INTERACTIVE TOOL

Try the SPC, Cp, Cpk and Ppk Calculator

Enter your values below, calculate, and download your result with formulas and assumptions.

10 CHAAAM tools

How to use this tool

  • Keep measurements in original time order; do not sort them by magnitude.
  • Paste readings or upload a local CSV/XLSX file and select the measurement column.
  • Enter the engineering specification limits, not control limits.
  • Review the I-chart, capability estimates and stability/sample warnings together.

Calculation method and formula

For individual observations, estimated within σ = mean moving range / 1.128. Cp = (USL − LSL)/(6σwithin), Cpk = min[(USL − mean),(mean − LSL)]/(3σwithin). Pp and Ppk use the overall sample σ.

WORKED EXAMPLE

In a ten-observation example with LSL 9.5 and USL 10.5, the illustrative dataset produces Cp ≈ 1.611 and Pp ≈ 1.414. Ten values are too few for confident process capability conclusions.

What the calculation does not cover

  • A capable process must also be statistically stable; Cp/Cpk alone are not a production acceptance decision.
  • I-MR assumes a specific within-sigma estimate and time ordering; subgroup or non-normal data require different methods.
  • At least 50 observations are recommended here; verify normality and measurement system adequacy independently.

Reference: NIST Engineering Statistics Handbook

Common questions

What is the difference between Cpk and Ppk?

Cpk uses a within-process variation estimate; Ppk uses overall sample variation, which can include shifts and longer-term changes.

Can I upload a spreadsheet privately?

CSV/XLSX input is read in the browser. XLSX requires downloading an Excel parsing library, but the file data are not sent to CHAAAM.

Engineering use: This calculator is a transparent estimate for its defined model; confirm units, applicability, standards and engineering decisions independently.