🛠️ Tool
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Free
Intermediate
Why we picked it Before you run anything, put your real numbers into this and see how many conversions per variant you would actually need. It takes your baseline conversion rate and the smallest effect you care about (the minimum detectable effect) and returns the sample size per group, which is the fastest honest way to learn whether a test can ever hit significance. If the number dwarfs your monthly traffic, that is your answer: skip the test and do something qualitative instead.
Sample Size Calculator (Evan's Awesome A/B Tools)
From evanmiller.org by Evan Miller
- Enter your baseline conversion rate and minimum detectable effect, and it returns the sample size needed per variation.
- Aiming to detect a small change requires a much larger sample, which makes the low-traffic trap concrete.
- It is a free, no-signup, single-page tool from a widely trusted source in A/B testing statistics.