Cp/Cpk Capability Calculator (C++)

Small C++ console app that computes Cp/Cpk capability indices from a 1-column text/CSV file.

Repo: codemark1210/cpktest • Toolchain: MSVC / Visual Studio 2022 • License: MIT

Why

In SPC/quality work you often need a quick, offline check of process capability. This tool prints n, mean, sd, Cp, Cpu/Cpl, Cpk from a measurement list.

Features

• CLI: --file <path> --lsl <num> --usl <num>
• Tolerates comma-separated lines
• Clear error messages (bad path, invalid limits, too few samples)
• Builds with Visual Studio (MSBuild); also works with GCC/Clang

Build & Run (Visual Studio)

1. Open the solution cpktest.sln.
2. Set Platform = x64, Configuration = Debug/Release.
3. Project → Properties → Debugging
   • Working Directory: $(ProjectDir)
   • Command Arguments:

     --file "$(ProjectDir)example_values.txt" --lsl 9.7 --usl 10.3
     

4. Run (Ctrl+F5). You should see an output like:

n                : 10
mean             : 10.0900
sd               : 0.0252
LSL              : 9.7000
USL              : 10.3000
Cp               : 1.9841
Cpu              : 2.7718
Cpl              : 1.5397
Cpk              : 1.5397
Capability grade : Good

File Format

example_values.txt (one value per line). Commas in a line are tolerated.

Theory (short)

$$ Cp = \frac{USL - LSL}{6\sigma} $$

$$ Cpk = \min!\left(\frac{USL - \mu}{3\sigma},, \frac{\mu - LSL}{3\sigma}\right) $$

Rules of thumb. Cpk < 1.0 (poor), 1.0–1.33 (marginal), 1.33–1.67 (good), ≥ 1.67 (excellent).

Roadmap

• --target / --tolerance argument pair
• CSV column selection
• Markdown report export
• Within-sigma (subgroup) option

License

MIT © 2025 codemark1210