# A/B Test Sample Size Calculator (Free) | George Peric
URL: https://www.cro-designer.com/tools/ab-test-sample-size-calculator
Last updated: 2026-10-26

> Free A/B test sample size calculator: enter your conversion rate and the lift you want to detect to see visitors per variant and how many days to run the test.

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Free A/B test sample size calculator

## How many visitors does your A/B test need?

Enter your current conversion rate and the smallest lift worth detecting. You get the visitors you need per variant, in total, and how many days the test should run.

### Your test

Use the conversion rate of the page you are testing, from the last few weeks.

Current conversion rate (%)

Minimum detectable effect (%)

The effect is

Relative (3% → 3.3%) Absolute (points)

Significance

Power

Variants incl. control

Daily visitors (optional)

Visitors per variant

–

Total visitors

–

Conversion rate to detect

–

Test duration

–

Get help running your tests

Free and private: everything is calculated in your browser. Need someone to plan, design and read your tests? See my A/B testing services.

### How to use the sample size calculator

Plan the sample size before the test starts, then don’t stop early. Enter the conversion rate of the page you are testing and the smallest lift that would be worth acting on. The smaller the lift you want to detect, the more visitors you need.

The calculator uses the standard two-sided test for two proportions, the same method most A/B testing tools use for planning. Significance is how often you accept a false winner (95% means 5%), and power is how often you catch a real effect of the size you entered.

### What if you don’t have enough traffic?

Test bigger changes, test closer to the money (checkout or form instead of a banner), use a metric with more conversions, or measure before and after instead. I explain each option in how many visitors you need for an A/B test, and how to roll a test out safely in how to ramp up an A/B test.

FAQ

### Frequently asked questions

What is a good minimum detectable effect?

The smallest lift that would change your decision and that your traffic can detect in two to six weeks. For many sites that is a 5% to 20% relative lift. Asking for a 1% lift usually needs far more traffic than you have.

Should I use 95% or 90% significance?

95% is the common default. 90% needs fewer visitors but accepts more false winners. For big, hard-to-reverse changes, use 95% or 99%.

Does this work with more than two variants?

Yes, the total and duration scale with the number of variants. With many variants, consider a stricter significance level, because more comparisons mean more chances of a false winner.

Can I stop the test as soon as it is significant?

No. Checking repeatedly and stopping at the first significant result inflates false winners. Run until the planned sample size, and at least two full weeks. See when to call an A/B test.

More free tools

### Other A/B testing calculators

#### CRO ROI calculator

See what a conversion lift is worth per month and per year for your store or lead gen site.

#### A/B test significance calculator

Is your winner real or just noise? Get the p-value, the lift and its confidence interval.

#### SRM checker

Check if your traffic split matches what you set. A mismatch means the test can’t be trusted.

Let’s talk

### Let’s make your next test a winner.

Tell me about your funnel. I’ll point out quick wins, tell you what I’d test first, and say honestly whether I’m the right fit.

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