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A/B Test Power Calculator
How long does your test actually need to run?
Standard sample-size calculators assume session-based statistics. For SaaS and ecommerce, where the same person counts as multiple sessions, that overstates the sample you need by 15-30%. This calculator runs both numbers so you can plan honestly.
Your test plan
Sample per variant
33,546
Total visitors needed
67,092
Days to complete
68days
Both methodologies side-by-side
Person-level
33,546
per variant
Session-based (VWO/Optimizely default)
41,932
per variant
Session-based calculators count the same user multiple times. For products with repeat visits or cross-device behavior, this overstates sample size by ~20%. Source: cross-session variance modeling on SaaS test panels.