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SPC Control Chart Calculator

This SPC calculator creates I-MR limits for individual readings or Xbar-R limits for complete rational subgroups. It is a screening aid for process stability, not a substitute for an approved control plan.

Run the calculation

Your data stays in this browser. No measurements are uploaded.

  1. 1 Enter data
  2. 2 Review results
  3. 3 Take action

Study inputs

Ready for your data

Complete the inputs or load the worked example. Results will include the primary decision, interpretation and supporting metrics.

Worked example

Load the ordered 20-reading example. Use subgroup size 1 for I-MR, then try subgroup size 5 to see how rational grouping changes the plotted statistic and limits.

Load the example in the calculator

Formula and methodology

  1. I-MR limits use the average two-point moving range and standard constants.
  2. Xbar-R limits use subgroup means, subgroup ranges and A₂/D₃/D₄ constants for subgroup sizes 2–10.
  3. Readings must remain in time order; only complete subgroups are analysed.
I chart: UCL/LCL = X̄ ± 2.66 MR̄MR chart: UCL = 3.267 MR̄Xbar chart: UCL/LCL = X̄̄ ± A₂R̄

How to interpret the result

  • A point beyond a calculated limit is a special-cause signal that needs investigation.
  • No point beyond the limits does not prove stability; review runs, trends, cycles and the sampling plan.
  • Control limits describe observed process behaviour and are not customer specification limits.

Common errors and limitations

  • Sorting measurements before charting them.
  • Using arbitrary subgroups that mix different process conditions.
  • Recalculating limits after every signal without investigating the cause.

Turn the calculation into an engineering decision

Share the drawing, process conditions and result context. AccuTouch can review the measurement, fixture, SPC or automation starting point.

Plan real-time SPC collection

Frequently asked questions

When should I use an I-MR chart?

Use it when meaningful rational subgroups are unavailable and measurements arrive one at a time.

Xbar-R or Xbar-S?

Xbar-R is common for small subgroups. Xbar-S is generally preferred for larger subgroup sizes because standard deviation uses more information.

Are control limits specification limits?

No. Control limits are calculated from process data; specification limits come from design or customer requirements.

Technical references

Use the current controlled standard or your organisation’s approved method where it differs from this screening tool.

Method content reviewed 13 August 2026. Independent technical approval is still required before production publication or use in a controlled quality procedure.