SPC control chart calculator — X-bar/R, p, np, c and u
Choose a chart type and paste measurements or defect counts in sample order.
Samples and control limits
| Sample | Observed | Centre | LCL | UCL | Signal 1/0 |
|---|---|---|---|---|---|
| Fill in the fields above — the result is computed on this page. | |||||
How do I use this page?
- Choose the chart that matches the data type.
- Paste observations in time order and set sample sizes.
- Review the limits, signals and chart.
What this page does not do: The tool flags individual points beyond 3-sigma limits. It does not test runs, trends or other Nelson rules and does not replace a sampling plan.
Further reading
Theory — what are SPC control charts?
SPC control charts separate the natural, common variation of a process from signals that are worth investigating. The X-bar/R chart handles measurements collected in equal-sized subgroups, p and np charts track the share of defective units, while c and u charts count nonconformities per sample. Control limits are calculated directly from the analyzed data, not from a predefined tolerance — they differ from tolerance limits, which are set by design, customer requirements or a standard rather than by the process itself.
Frequently asked questions
How is a control limit different from a specification?
Does a point inside the limits prove good quality?
See also
A/B test calculator
Compare two proportions in an A/B test. Calculate conversion rates, difference, uplift, z score, p value and c
A/B test planning — sample size, MDE and duration
Plan a two-proportion experiment, estimate detectable effect and traffic duration.
ACF and PACF autocorrelation calculator
Calculate ACF and PACF across time-series lags. Review the chart, reference bounds and coefficient table.