A/B Test Significance Calculator
Enter visitors and conversions for both variants. You'll get the p-value, the relative lift, and a straight answer on whether the difference clears the usual 95% bar.
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p = 0.0055 · 99.4% confidence
Two-tailed pooled z-test. A p-value is the chance of seeing a difference this large if the two variants were actually identical — not the chance that B is better.
How significance is calculated
A pooled two-proportion z-test. The pooled rate p̂ = (x₁ + x₂) ÷ (n₁ + n₂), the standard error is √(p̂(1−p̂)(1/n₁ + 1/n₂)), and z is the difference in rates divided by that standard error.
The p-value is two-tailed: 2(1 − Φ(|z|)). One-tailed tests halve the p-value and flatter whichever variant happens to be ahead, so they're not offered here.
A p-value is the probability of a difference at least this large if the two variants were genuinely identical. It is not the probability that B beats A.
Questions people ask
How long should I run a test?
Decide the sample size before you start and don't stop early because a result looks good — checking repeatedly and stopping at the first significant reading is the most common way to produce a false positive.
My result isn't significant. Does that mean the variants are the same?
No. It means you don't have enough evidence to distinguish them. Absence of significance is not evidence of equivalence, especially at small samples.
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