Evan's Awesome A/B Tools (home):

Sample Size Calculator  |  Chi-Squared Test  |  Sequential Sampling  |  2 Sample T-Test  |  Survival Times  |  Count Data

Question: How many subjects are needed for an A/B test?

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Baseline conversion rate: %
Minimum Detectable Effect: %
10.2% – 13.2%
The Minimum Detectable Effect is the smallest effect that will be detected (1-β)% of the time.
Conversion rates in the gray area will not be distinguishable from the baseline.
Sample size:
per variation
Statistical power 1−β:
Percent of the time the minimum effect size will be detected, assuming it exists
Significance level α:
Percent of the time a difference will be detected, assuming one does NOT exist

See also: How Not To Run an A/B Test

If you like this, check out Wizard — the easy Mac statistics app.