A/B Test Sample Size Calculator — free online
You changed a button color, or rewrote a headline, and now you're watching the numbers. One version looks a little better. But is it really better, or is it just luck with a small group of visitors? Stopping a test too early is one of the easiest mistakes to make. We added a free tool that tells you when you have enough data — A/B Test Sample Size — right in your browser.
What it does
You type in a few numbers: your current conversion rate, the smallest change you'd care about, the statistical power you want, and your confidence level. The tool then works out how many visitors each version of your test needs before you can trust the answer. That's the whole job — a clear target number, so you know how long to wait.
Nothing leaves your device
It all runs in your browser. The numbers you enter stay on your screen — nothing is uploaded, nothing is saved on a server. Close the tab and it's gone.
How to use it
- Open A/B Test Sample Size and type in your current conversion rate.
- Set the smallest improvement worth detecting, your power, and your confidence level.
- Read the visitors-per-version number, and run your test until you reach it.
It's free with no limit. There are more small tools on the Apps page.