We Recommend the Following A/B Test Sample Size Calculators

Evan Miller’s Sample Size Calculator

http://www.evanmiller.org/ab-testing/sample-size.html

Like most of the tools on Evan Miller’s site, we fully recommend the Sample Size Calculator. This tool has the basics of what you need to calculate a valid sample size and nothing you don’t need. Just enter your statistical power, significance level, baseline conversion rate, MDE, and the tool will take it from there.

Evan Miller Sample Size Calculator
Optimizely’s Sample Size Calculator

https://www.optimizely.com/resources/sample-size-calculator/

This sample size calculator is pretty straightforward. The only drawbacks are that you have to calculate your MDE relatively and you can’t manually set your statistical power.

Another consideration for this calculator being Optimizely uses it own compute logic to arrive at recommended sample size, rather than traditional sample formulas. This is based on their documented work with the “Stats Engine” methodology baked into the testing platform.

Optimizely Sample Size Calculator