A/B test sample size calculator
Before you launch an experiment, find out how many visitors each variant needs — and how long that will take — for a statistically trustworthy result.
Experiment setup
Two variants: control (A) and challenger (B), split 50/50.
The control's current rate, e.g. 3 for 3%.
Relative lift you care about — 15 means detecting 3% → 3.45%.
Chance the detected lift is not a false positive. 95% is the norm.
Chance of detecting the lift if it really exists. 80% is the norm.
Total traffic entering the test (both variants). Used to estimate duration.
Required sample
Per variant
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Total sample
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Detectable change
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Assumes a two-sided z-test on proportions with an even 50/50 split.
Stopping the test early, before the sample is reached, inflates false positives.