Control limit calculator
Give it subgroups or individual readings and it returns the control limits for the chart, the range or sigma chart beneath it, and every constant it used to get there.
Nothing you paste leaves this calculation. It is not stored, not logged and not sent anywhere else — the numbers are computed and returned, and that is all.
Process chart
Dispersion chart
Constants used
How this is calculated
Control limits describe what the process does when nothing unusual is happening. They come from the data's own short-term variation — never from the specification, and never from the spread of the whole history.
X̄–R UCL = x̄̄ + A₂R̄ R chart: UCL = D₄R̄, LCL = D₃R̄ X̄–s UCL = x̄̄ + A₃s̄ s chart: UCL = B₄s̄, LCL = B₃s̄ I–MR UCL = x̄ + 2.66·MR̄ MR chart: UCL = 3.267·MR̄ c UCL = c̄ + 3√c̄ LCL = max(0, c̄ − 3√c̄)
The part most people get wrong
Control limits are not specification limits. One is what the process does, the other is what the drawing demands. A process can be perfectly in control and entirely incapable, and putting the tolerance on the chart as if it were a limit hides exactly that.
And limits should be frozen. Recomputing them from whatever is on screen — the single most common mistake in SPC dashboards — means a drifting process quietly drags its own limits along with it and never signals. Compute them once from a stable window, lock them, and judge everything after against those.
See the full table of constants for every subgroup size from 2 to 25.
Want this to keep itself up to date?
A calculator answers for the data you pasted. A control chart answers for the data your line produced this morning — limits frozen at a baseline you locked, rules evaluated on every new reading, an alert when one trips.
Other free tools
- Cp / Cpk calculator — Paste measurements — or type a mean and a sigma — with your tolerance, and get Cp, Cpk, Pp, Ppk, the sigma level and the expected parts per million out of spec.
- Control chart generator — Paste a column of numbers, or rows of subgroups, and get a real control chart: limits from the data, Nelson rules 1–4 evaluated, out-of-control points marked.
- Control chart constants — The whole table, n = 2 to 25, with the formula each constant belongs to. The same values the charts on this site are computed from.