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A/B test sample size calculator

Enter your baseline rate and minimum detectable effect and get visitors per variant, run-time estimates from daily traffic, and the full formula trace.

Enter a baseline and an effect to size the test.

The plan is a normal approximation of the two-proportion comparison, computed before the test runs. Real tests also lose visitors to mid-experiment filtering, cross-device doubles, and peeking at partial results, and those losses push the honest number up. Sequential and Bayesian designs need different sizing than this fixed-horizon plan, and the tool says so rather than pretending to cover them.
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