Set your baseline
Enter the current conversion rate of the control variant, plus the smallest lift you want the test to reliably catch.
A/B Test Sample Size Calculator
Enter your baseline conversion rate and the smallest change worth detecting, and get the sample size and estimated duration your test needs.
A 10% relative lift on a 5% baseline targets 5.5%.
–Per variant
–Total (both variants)
–Estimated days
How it works
Choose confidence and power
Higher confidence and power both reduce the chance of a wrong call, but require a larger sample size to reach.
Read your target
Get the sample size needed per variant, the total, and — if you enter daily traffic — how many days the test should run.
FAQ
What is minimum detectable effect (MDE)?
The minimum detectable effect is the smallest change in conversion rate you want your test to be able to reliably detect. A smaller MDE requires a larger sample size.
What happens if I stop the test early?
Stopping before reaching the calculated sample size, or checking results repeatedly and stopping as soon as they look significant, inflates your false positive rate. Decide on a sample size in advance and let the test run to completion.
Should I use a one-sided or two-sided test?
Two-sided tests detect a change in either direction and are the safer default. One-sided tests only detect an increase (or only a decrease) and need a smaller sample, but you should only use one if you truly do not care about detecting a change in the other direction.