Free calculator
A/B test duration calculator
How many days does a test need to run at your current traffic? Combines the sample- size formula with your daily visitors and traffic split.
Your current control rate.
Relative lift you want to catch.
95 standard.
80 standard.
Visitors reaching the page you're testing per day.
100 if every visitor is included. 50 for a 50/50 split with control-only cohorts, etc.
Duration
13 days
Visitors per variant31,235
Total sample62,470
Weeks1.9
Runs shorter than 7 days under-sample your weekday/weekend traffic mix — plan for one full week minimum regardless of what the calculator says.
Planning rules
- 7-day minimum, always. Even if the calculator says 3 days, weekday and weekend traffic don't behave the same. Under-sampling one cohort skews the result.
- Two-week ceiling for iterative tests. If a test needs more than two weeks at your traffic, pick a bigger MDE or a higher-traffic page — the risk of confounds compounds every extra week.
- Split matters. Half your traffic on control-only means you need double the days.
- Estimate baseline honestly. Use the last 30 days of the actual page's conversion rate, not the aggregate funnel average.