Sample Size Calculator  ChiSquared Test  Count Data  Survival Times  2 Sample TTest
Question: How many subjects are needed for an A/B test?
%  
The Minimum Detectable Effect is the smallest effect that will be detected (1β)% of the time. 

Conversion rates in the gray area will not be distinguishable from the baseline. 
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