A/B Test Duration Calculator

An A/B test duration calculator answers the question a sample size number alone cannot: how many calendar days will it actually take to collect that sample given your real traffic. It uses the same two-proportion sample size formula as our A/B test sample size calculator, then divides by your daily traffic across variations and applies a 14 day floor.

Sample size versus duration

Sample size tells you how many visitors each variation needs to detect your target lift reliably. Duration takes that same sample size and divides it by your actual daily traffic, split across your variations, to tell you how many calendar days that traffic will take to arrive. Two sites needing the identical sample size can have very different durations if one gets far more daily visitors than the other.

Why a 14 day minimum

Even a test that mathematically reaches its sample size in a handful of days is commonly extended to at least one to two weeks so it captures a full weekly cycle, since visitor behavior and conversion rate often differ meaningfully between weekdays and weekends. Cutting a test short before that cycle completes risks a result skewed by whichever days happened to be included, which is why this calculator never returns fewer than 14 days even when the raw math needs less.

What to do about a duration that is too long

A long duration usually means your minimum detectable effect is too small for your traffic level. Consider testing for a larger, more meaningful lift, testing a bigger change likely to move the needle further, or concentrating traffic on fewer, higher-impact pages rather than spreading a test across low-traffic ones. Whatever the duration comes out to, commit to it in advance and avoid stopping the test early the moment it looks significant, since checking repeatedly and stopping at the first good-looking result inflates your false positive rate well beyond your chosen confidence level.