Sample Size & Precision Calculator

See what your sample can detect—or calculate the sample required for the difference your study needs to find.

Study assumptions

Group A raw sampleGroup B raw sampleEnter the actual sizes when the comparison groups are unequal.Expected result (%)Percent selecting the outcome. Use 50% for the conservative case.Confidence levelStatistical powerDifference you need to detect (percentage points)Percentage-point difference between equal-sized groups.Weighting efficiency (%)Use 100% before weighting or when no estimate is available.

Planning readout

Effective sample: Group A: 500
Effective sample: Group B: 500
Margin of error: Group A: ±4.4 pts
Margin of error: Group B: ±4.4 pts
Minimum detectable difference: 8.9 pts
Effective sample needed per equal-sized group: 614
Raw sample needed per equal-sized group: 614

Two different questions: margin of error describes one estimate. Minimum detectable effect describes the comparison between the two entered groups.

How to use this

Size against the primary outcome and the smallest group you need to report. Weighting efficiency reduces the raw base to the effective base used for inference.

Read the sample-size methodology for formulas, assumptions, unequal groups, and worked examples.

Need help sizing or fielding your study?

Share the decision, audience, and precision you need. An MX8 Labs researcher can help turn the calculation into a workable study.

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