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eng.spc-control-chart-limits Calculator
Calculates Statistical Process Control (SPC) control limits (UCL, LCL) for X-bar, R, and p charts from sample data. Control limits at ±3σ capture 99.73% of natural variation — points outside limits signal special cause variation requiring investigation.
Inputs
Process Mean
Arithmetic average. Sensitive to outliers — if your data has extreme values, the median may be more representative.
Process Std
Reference formula or conversion factor shown for context.
Sample Size
Number of data points collected. Larger samples give narrower confidence intervals. Rule of thumb: 30+ for most statistical tests.
Results
UCL (upper control limit)
Sample size or count used in the calculation.
LCL (lower control limit)
Sample size or count used in the calculation.
σ of sample means
Arithmetic average — sum divided by count. Simple and familiar, but pulled by outliers. If your data contains extreme values, the median may be more representative.
3-sigma rule
Reference formula or conversion factor shown for context.
A₂ factor (n=
A dimensionless multiplier applied in the calculation.
capability Cp (if spec ±3σ)
Reference formula or conversion factor shown for context.