Cp / Cpk Calculator

A simple Flask web app for process capability analysis.

This tool lets you upload measurement data from a CSV file, enter USL and LSL, and instantly calculate:

• Cp
• CPU
• CPL
• Cpk

It also shows a histogram, capability interpretation, sample datasets, and practical next-step guidance for manufacturing and quality teams.

👉 Live demo: tools.ryo-aihub.com

Why This Project

Process capability checks are often done in spreadsheets, which makes quick reviews harder than they need to be.

This app gives you a lightweight browser-based workflow for:

• checking whether a process is capable
• seeing whether the process is off-center
• comparing sample scenarios before using real data
• reviewing results in a way that is easier to explain to teammates

If you just want to answer "Is this process actually healthy?" without building an Excel sheet first, this tool is for you.

What This Tool Is For

This project is designed for people who want to quickly check whether a process is capable and whether it is centered within specification limits.

• Cp tells you how much process spread fits inside the spec width.
• Cpk tells you how capable the process actually is after considering centering.

A process can have a high Cp but still a poor Cpk if the mean is shifted toward one limit. That is why Cpk is often the more practical number when reviewing process risk.

Features

• Upload a CSV file and use the first column as measurement data
• Use built-in sample datasets for quick testing
• Calculate Cp, CPU, CPL, and Cpk
• Visualize the distribution with spec limits and mean
• Get a plain-language capability interpretation
• Review recommended next checks based on the result
• Run everything locally with Python and Flask

At A Glance

• Input: sample data or your own CSV
• Required values: USL and LSL
• Output: capability metrics, histogram, interpretation, and suggested next checks
• Best for: manufacturing, quality, process engineering, and teaching basic capability analysis

How To Use

1. Open the live tool or run the app locally.
2. Choose sample data or upload a CSV file.
3. Enter USL and LSL.
4. Click Calculate capability.
5. Review the metrics, chart, and interpretation.

CSV Format

The app reads the first column of the CSV as numeric measurement values.

Example:

value
9.8
10.1
9.9
10.0
10.2

Formula Reference

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

Quick Python Example

def calc_cp_cpk(mean, std, lsl, usl):
    cp = (usl - lsl) / (6 * std)
    cpu = (usl - mean) / (3 * std)
    cpl = (mean - lsl) / (3 * std)
    cpk = min(cpu, cpl)
    return cp, cpk


mean, std, lsl, usl = 10, 1, 8, 12
cp, cpk = calc_cp_cpk(mean, std, lsl, usl)
print(f"Cp: {cp:.2f}, Cpk: {cpk:.2f}")

Run Locally

Python 3.11 is recommended for local development and deployment.

python -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
./.venv/bin/python app.py

Then open http://127.0.0.1:8000.

Example Interpretation

• High Cp and high Cpk: the process is tight and well-centered
• High Cp and low Cpk: variation may be acceptable, but the process is shifted
• Low Cp and low Cpk: the process spread itself is too wide for the spec

This side-by-side view is one of the main reasons engineers review Cp and Cpk together instead of looking at only one metric.

FAQ

What is the difference between Cp and Cpk?

Cp looks at spread only. Cpk looks at spread plus centering.

What is a good Cpk value?

Many teams use 1.33 as a practical target, but the right threshold depends on customer requirements, product risk, and process stability.

What kind of CSV can I upload?

Any CSV where the first column contains numeric measurement values. Non-numeric values are ignored.

Project Structure

app.py                  Flask entry point
web/routes.py           Routes and page rendering
services/analysis.py    Capability calculations and chart helpers
services/calculator.py  Form handling and calculator flow
data/sample_catalog.py  Built-in sample datasets
templates/              HTML templates
static/                 CSS assets

Who This Is Useful For

• Manufacturing engineers
• Quality engineers
• Six Sigma practitioners
• Process engineers
• Students learning process capability

Contributing

Issues and pull requests are welcome. If you find a bug, want to improve the UX, or want to expand the capability analysis content, feel free to open a discussion.

License

MIT License. See LICENSE.