Free toolsProcess capability calculator
Process capability calculator.
Paste your measurements and specification limits. Get Cp, Cpk, Pp and Ppk, the expected defect rate and a verdict you can take into a meeting. It all runs in your browser, so your data never leaves your computer.
Results
Enter your readings and at least one limit, or try the sample: 50 bore diameters on a part toleranced Ø25.000 ±0.020 mm, measured in subgroups of five.
Cpk, short term
0.00
Capable
Cp
0.00
Ppk, overall
0.00
Pp
0.00
- Readings
- Mean
- Sigma, within
- Sigma, overall
- Range
- Cpl and Cpu
- and
- Out of spec, observed
- Out of spec, expected
- ppm short term, ppm overall
How to read the numbers
Capability compares the spread of your process with the width of the tolerance. The four indices answer slightly different questions.
- Cp
- Could the process fit inside the tolerance if it were perfectly centred? It uses short-term variation and ignores where the mean sits.
- Cpk
- Does it fit where it actually runs? The distance from the mean to the nearer limit, divided by three short-term sigma. This is the number most customers ask for.
- Pp and Ppk
- The same questions using overall variation, which includes drift between setups, shifts and material lots. A big gap between Cpk and Ppk means the process isn't stable over time.
- What is a good Cpk?
- Below 1.00 the process is not capable. 1.00 to 1.33 is marginal. 1.33 is the common minimum for production, and 1.67 is typical for new or safety-critical characteristics.
- Short-term sigma
- Estimated from the average moving range for individual readings, or the average subgroup range for subgroups of 2 to 10. Overall sigma is the ordinary sample standard deviation.
- Before you trust it
- Cpk assumes the process is stable, the data are roughly normal and the gauge is good enough. A capable-looking number from an unstable process or a poor gauge is not real.
Need a study you can hand to a customer?
The calculator gives you the indices. A capability study tells you whether to believe them: we check gauge capability, stability over time and the shape of the distribution, then write up what the numbers mean and what to change.