A/B Test Sample Size Calculator — Free, Two-Proportion Z-Test
Free A/B test sample-size calculator using the two-proportion z-test. Set baseline CVR, MDE, and confidence level — get the per-variant sample size needed.
A/B Test Sample Size Calculator
Per variant
31,196
Total (A + B)
62,392
Target new rate
5.50%
Assumes 80% statistical power. Smaller minimum-detectable-effect (MDE) = much larger sample. Detecting a 5% lift on a 2% baseline takes ~30K visitors per variant.
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Work out the sample size for an A/B test
Enter your baseline conversion rate, the minimum detectable effect (MDE) you care about, and your confidence level — get the number of visitors needed per variant before the result is trustworthy. It uses the two-proportion z-test that powers most experimentation platforms, including Adobe Target, Optimizely and VWO.
The inputs that matter
- Baseline CVR — your current conversion rate.
- MDE — the smallest lift worth detecting (a smaller MDE needs a much bigger sample).
- Confidence & power — usually 95% confidence and 80% power.
Why sample size matters
Calling a test early is the most common A/B testing mistake — early "winners" often regress to the mean. Fix your sample size up front, run until you hit it, and only then read the result. Always-on programmes can use sequential testing to peek safely, but a fixed-horizon sample size is the reliable default. Everything is computed in your browser.
How to use the A/B Test Sample Size Calculator
Takes about a minute. No signup, no download, your data stays in your browser.
- 1Open the tool. Scroll up to the A/B Test Sample Size Calculator above — it loads instantly in your browser, no install needed.
- 2Enter your values. The fields come pre-filled with realistic defaults so you can see how it works — replace them with your own numbers.
- 3Read the result. The output updates instantly. Copy or share it — nothing is uploaded to a server, everything stays on your device.
Frequently asked questions
Common questions about the A/B Test Sample Size Calculator.
Is the A/B Test Sample Size Calculator free to use?
Yes. The A/B Test Sample Size Calculator on 7BusyBoss is completely free — no signup, no paywall, no usage cap. We do recommend paid alternatives in a sidebar card, but the core tool stays free.
Do I need to sign up to use the A/B Test Sample Size Calculator?
No. The A/B Test Sample Size Calculator works instantly in your browser — no account needed. Signing in with Google is optional and only unlocks features like saving favorites and higher AI build limits.
Does the A/B Test Sample Size Calculator work on mobile?
Yes. Every tool on 7BusyBoss is mobile-responsive and works in any modern browser — Chrome, Safari, Firefox, Edge — on phones, tablets and desktops.
Is my data safe when I use the A/B Test Sample Size Calculator?
Yes. The A/B Test Sample Size Calculator runs entirely in your browser — your inputs never leave your device unless the tool explicitly fetches public data (e.g. currency rates). We don't store the values you type.
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