What confidence level should I choose for my A/B test?

A 95% confidence level is standard in most fields, meaning there is a 95% probability that the true conversion rate falls within the calculated interval. Higher levels like 99% require larger sample sizes.

How does the minimum detectable lift affect the required sample size?

The smaller the lift you want to detect, the larger the sample size needed. If you only care about large improvements, you can reduce the required visitors per variant.

What does margin of error mean in this context?

Margin of error expresses the range above and below the sample statistic. For example, a 5% margin of error with 50% conversion means the true rate is likely between 45% and 55%.

Can I use this calculator for finite populations?

The calculator includes a population size field; leave it blank if your population is large or unknown. Entering a specific population size will adjust the sample size downward if the sample is a significant fraction of the total.

StatisticsTool Review

Sample Size Calculator

Provided byCalculator.netcalculator.net

This free sample size calculator determines the sample size required to meet a given set of constraints. Also, learn ...

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About this tool

What Sample Size Calculator does

A sample size calculator that determines the minimum number of observations needed to meet specific statistical constraints for A/B testing or survey results. Users input a baseline conversion rate, the minimum detectable lift they wish to achieve, confidence level, and margin of error to calculate the required sample size. The tool outputs the necessary visitor count per variant to detect a statistically significant difference. It also includes a separate margin of error calculator for existing data. The site provides standard confidence level options and a note suggesting 50% population proportion when no prior estimate exists, along with a link to related standard deviation and probability calculators for further statistical exploration.

Step by step

How to use the Calculator.net Sample Size Calculator

1
Select the desired confidence level from the dropdown menu (e.g., 95%)
2
Set the margin of error percentage to define the acceptable range of error
3
Enter the estimated population proportion or use the default 50% if unknown
4
Click Calculate to view the minimum sample size required per group

Is it right for you

Best for

Marketers and researchers running A/B tests who need to determine the minimum number of visitors per variant to reliably detect a specified performance lift.

Limitations

Results are estimates based on normal approximation and may not account for all study designs
Requires user to define the minimum detectable lift and confidence level; defaults may not fit every use case
Population proportion default of 50% may overestimate sample size if prior data suggests a different proportion

Questions

Sample Size Calculator FAQ

What confidence level should I choose for my A/B test?: A 95% confidence level is standard in most fields, meaning there is a 95% probability that the true conversion rate falls within the calculated interval. Higher levels like 99% require larger sample sizes.
How does the minimum detectable lift affect the required sample size?: The smaller the lift you want to detect, the larger the sample size needed. If you only care about large improvements, you can reduce the required visitors per variant.
What does margin of error mean in this context?: Margin of error expresses the range above and below the sample statistic. For example, a 5% margin of error with 50% conversion means the true rate is likely between 45% and 55%.
Can I use this calculator for finite populations?: The calculator includes a population size field; leave it blank if your population is large or unknown. Entering a specific population size will adjust the sample size downward if the sample is a significant fraction of the total.

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