Use our free Control Limit Calculator to determine the Upper Control Limit (UCL) and Lower Control Limit (LCL) for various statistical process control charts. Monitor and improve process stability, identify special cause variation, and enhance quality with this essential tool for SPC.
Formula:
The Upper Control Limit (UCL) and Lower Control Limit (LCL) for X-bar and R charts are derived using the following formulas:
- For X-bar Chart:
- UCLX̄̄ = X̄̄ + A2R̄
- LCLX̄̄ = X̄̄ - A2R̄
- For R Chart:
- UCLR̄ = D4R̄
- LCLR̄ = D3R̄
- X̄̄ (X-double-bar): The grand average of all subgroup averages (the process average).
- R̄ (R-bar): The average of all subgroup ranges (a measure of process variation).
- A2, D3, D4: Control chart constants, which depend on the subgroup size (n) and are looked up from standard control chart constant tables.