Track subgroup dispersion using reliable range control limits. Analyze samples, chart ranges, and flag variation. Download reports and validate process consistency with clear calculations.
This example uses six subgroups with five observations each. You can paste the same pattern into the calculator to test the control limits workflow.
| Subgroup | Observation 1 | Observation 2 | Observation 3 | Observation 4 | Observation 5 | Range |
|---|---|---|---|---|---|---|
| 1 | 12.1 | 12.4 | 12.3 | 12.2 | 12.0 | 0.4 |
| 2 | 11.9 | 12.2 | 12.0 | 12.1 | 12.3 | 0.4 |
| 3 | 12.5 | 12.3 | 12.4 | 12.2 | 12.6 | 0.4 |
| 4 | 12.0 | 12.1 | 11.8 | 12.2 | 12.3 | 0.5 |
| 5 | 12.4 | 12.5 | 12.2 | 12.3 | 12.1 | 0.4 |
| 6 | 11.8 | 12.0 | 12.1 | 11.9 | 12.2 | 0.4 |
Subgroup range: Ri = max(x) - min(x)
Average range: R̄ = sum of subgroup ranges / number of subgroups
Lower control limit: LCLR = D3 × R̄
Center line: CLR = R̄
Upper control limit: UCLR = D4 × R̄
Estimated process sigma: σ = R̄ / d2
D3, D4, and d2 depend on subgroup size. The calculator automatically selects the proper constants for subgroup sizes from 2 through 25.
Enter one subgroup per line in the observations box. Keep every subgroup the same size. Separate values with commas, spaces, semicolons, or vertical bars.
Choose the number of decimal places that fits your measurement system. Add a process name and chart title if you want cleaner exports.
Click the calculate button. The page shows R-bar, control limits, sigma estimate, subgroup detail, and a plotted range chart above the form.
Download the CSV file for spreadsheet review or the PDF file for audit documentation, meeting notes, or quick reporting.
An R-chart monitors within-subgroup variation. It shows whether sample spread stays stable over time, which helps verify measurement consistency and short-term process dispersion.
Use an R-chart when subgroup sizes are small, usually between 2 and 10. It is simple, fast, and widely used with X-bar and R-chart methods.
Control chart constants depend on subgroup size. If subgroup sizes change, D3, D4, and d2 change too, which makes one set of limits invalid.
A point above the UCL suggests unusually high variation inside that subgroup. Investigate tool wear, setup drift, incoming material changes, or measurement problems.
Yes. For smaller subgroup sizes, D3 equals zero, so the lower control limit becomes zero. That is normal for many practical R-chart applications.
Yes. It uses sigma = R-bar divided by d2. This estimate is useful for understanding short-term dispersion, although capability studies need added context.
Only after you identify a clear special cause and document it. Recalculate limits after the abnormal condition is corrected and excluded from baseline behavior.
Yes. R-charts commonly accompany X-bar charts. First confirm variation is stable with the R-chart, then interpret subgroup means using the X-bar chart.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.