Sample Size Calculator
How many visitors do you need per variant before you can trust your A/B test results? Get the exact number – before you start testing.
Set your baseline CVR and MDE to see the sample size you need.
Why sample size matters before you test
The most common A/B testing mistake: stopping a test early because it "looks like" it's winning. Without a pre-calculated sample size, your results are meaningless – you're just watching noise. This is called peeking, and it inflates false positive rates to 25–30%.
Calculate your required sample size before you start. Then commit to running until you hit it – regardless of intermediate results.
Understanding the inputs
Your current conversion rate. The lower your CVR, the more traffic you need to detect changes. At 1% CVR you need ~4× more visitors than at 4% CVR.
The smallest uplift worth detecting. Setting MDE too low means you need enormous traffic. For most eCommerce tests, 10–20% relative MDE is practical.
At 80% power, if there's a true effect you'll detect it 8 out of 10 times. Higher power = more traffic needed, but fewer missed wins.
Running too many inconclusive tests?
Most brands don't have enough traffic to test everything. We prioritize the highest-impact experiments so every test counts.