Enter process data
Use raw measurements or summary statistics. The calculator supports one-sided and two-sided specifications.
Example data table
This sample shows a simple measured characteristic against a nominal value near 50.00 mm.
| Sample | Measurement (mm) | Status |
|---|---|---|
| 01 | 49.96 | Pass |
| 02 | 50.02 | Pass |
| 03 | 49.99 | Pass |
| 04 | 50.05 | Pass |
| 05 | 49.97 | Pass |
| 06 | 50.01 | Pass |
| 07 | 50.04 | Pass |
| 08 | 49.95 | Pass |
| 09 | 50.07 | Pass |
| 10 | 50.00 | Pass |
| 11 | 49.94 | Pass |
| 12 | 50.03 | Pass |
Formula used
Two-sided spread indices
Cp = (USL − LSL) / (6 × Within Sigma)
Pp = (USL − LSL) / (6 × Overall Sigma)
These indices compare tolerance width with total process spread. They ignore centering.
Centering-sensitive indices
Cpu = (USL − Mean) / (3 × Within Sigma)
Cpl = (Mean − LSL) / (3 × Within Sigma)
Cpk = minimum of Cpu and Cpl
Ppu = (USL − Mean) / (3 × Overall Sigma)
Ppl = (Mean − LSL) / (3 × Overall Sigma)
Ppk = minimum of Ppu and Ppl
Target-sensitive index
Cpm = (USL − LSL) / [6 × √(Overall Sigma² + (Mean − Target)²)]
Cpm penalizes both variation and drift away from target.
Defect prediction
Expected PPM uses the normal distribution and overall sigma.
For raw data mode, within sigma is estimated from average moving range divided by 1.128.
How to use this calculator
- Enter at least one specification limit. Enter both limits for full Cp, Cpk, Pp, and Ppk analysis.
- Add a target if you want target deviation and Cpm.
- Choose one input method. Either paste raw measurements or enter mean and sigma values manually.
- Submit the form. The result block will appear above the form, directly below the header.
- Review spread indices, centering indices, expected PPM, and the capability chart.
- Export the results or the example table using the CSV and PDF buttons.
FAQs
1. What does capability ratio mean?
It shows how well process variation fits inside specification limits. Higher values mean the process has more room before producing nonconforming output.
2. What is the difference between Cp and Cpk?
Cp only compares spread with tolerance width. Cpk also checks centering, so it drops when the process mean shifts toward one limit.
3. What is the difference between Pp and Ppk?
Pp and Ppk use overall sigma, which includes long-term variation. They are helpful for understanding actual, sustained performance rather than short-term potential.
4. What value is usually considered acceptable?
Many teams use 1.33 as a common minimum target. Critical applications may require 1.67 or higher, depending on risk, regulation, and customer expectations.
5. Can I use raw measurements instead of summary values?
Yes. Paste at least two measurements. The calculator will estimate mean, overall sigma, and within sigma automatically from the entered sequence.
6. Why are within sigma and overall sigma different?
Within sigma reflects short-term variation, while overall sigma captures broader process drift over time. A large gap often signals instability or shifting performance.
7. Why does a centered target matter?
A process can have low variation yet still run close to one limit. Target-based measures help show how far the mean has moved from the preferred setting.
8. Does this replace control charts?
No. Capability analysis assumes the process is stable. Use control charts first to confirm statistical control, then evaluate capability with confidence.