Approximate users needed per group to detect a conversion-rate lift at 95 percent confidence and 80 percent power. Runs in your browser; nothing you type is sent anywhere.

Baseline conversion rate (%)Minimum detectable lift (% relative)

Formula

Incremental lift equals the conversion rate of the exposed group minus the conversion rate of the control group, divided by the control group's rate. If 2.4 percent of exposed users converted and 2.0 percent of a randomized holdout did, lift is 20 percent and incremental conversions are 0.4 percent of the exposed group. Incremental CPA equals spend divided by those incremental conversions.

The groups must be randomized before exposure, the test must be sized in advance to detect the expected lift, and the result carries a confidence interval that should be reported with it. Geo tests replace user groups with markets and use the same arithmetic.

Typical ranges are on the benchmarks pages; definitions in the glossary.