Free calculator
Sample size calculator for A/B tests
Estimate the visitors per variant you need to catch a minimum detectable effect at a given confidence and power. Two-proportion formula, industry-standard defaults.
Your current control rate. 5 = 5%.
Relative lift you want to catch. 10 = detect ≥ +10% over baseline.
95 is standard. Higher = harder to reject the null.
80 is standard. Chance of detecting a real effect.
Visitors per variant
31,235
Total across both
62,470
Reach this per variant before you trust the result. Halt the test earlier and you risk a false positive.
Rules of thumb
- Sample size scales inverse-quadratically with MDE. Halving your MDE target quadruples the sample size you need.
- Low baselines are expensive. A 1% baseline needs ~25× the traffic of a 20% baseline for the same relative MDE.
- 95% confidence + 80% power are the defaults everyone uses. Higher = tighter but longer.
- Round up. Never launch a test with fewer visitors than the calculator says.
Related
- Test-duration calculator → Feed this sample size in with your daily traffic to get days.
- Significance calculator → Once the run finishes, compute the p-value.
- Glossary of A/B testing terms