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Sample Size Calculator.
Work out how many visitors an A/B test needs per variant, or how many survey responses you need for a margin of error, with test duration.
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How to use Sample Size Calculator
Choose A/B test or survey. For a test, enter the baseline rate, the smallest effect you care about, power and daily traffic. For a survey, set the margin of error and, if known, the population size.
How it works
A/B: the standard two-proportion formula. Survey: n₀ = z² p (1 − p) ÷ e² with the finite population correction.
Example
With a 5 % baseline and a 10 % relative lift at 95 % confidence and 80 % power, you need 31,234 visitors per variant; a survey at ±5 % needs 385 responses.
Questions about Sample Size Calculator
Is Sample Size Calculator free, and do I need an account?
Yes. This tool is free to use in your browser without an account. Processing takes place on your device.
What are the limits and what should I check?
Assumes random samples. More than two variants use a Bonferroni-corrected significance level. Everything is calculated in your browser; nothing you type is uploaded. Review the result before using it in your work.
Is my input sent to a server?
Your input is processed in your browser and is not uploaded by this tool. Files and pasted text are cleared when the page is refreshed. Normal website requests are still required to load the page and its libraries. Read the privacy explanation.