Sample Size Calculator  ChiSquared Test  TwoSample TTest  Poisson Means  Survival Times
Question: How many subjects are needed for an A/B test?
%  
The MDE is the smallest difference that can be detected with the prescribed number of subjects. 

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