Minimum detectable effect (MDE) calculator
If your traffic is fixed, work backwards: see the smallest lift a test of a given length can reliably detect.
What you can run
How long you're willing to run the test.
Minimum detectable effect
19%
In this window you can reliably detect a lift from 3% to 3.57% or larger.
Visitors per variant
14,000
Detectable target rate
3.57%
What minimum detectable effect means
Minimum detectable effect (MDE) is the smallest lift your test can reliably catch. The sample size calculator starts from the lift you want and tells you the traffic you need. This one flips that around: it takes the traffic you actually have over a fixed window and tells you the smallest lift that traffic can detect.
Here n is the visitors per variant your window provides, p is the baseline conversion rate, and zα and zβ come from your confidence and power. The result is a relative lift: a 12% MDE on a 3% baseline means you can detect a move to roughly 3.36% or more.
When to reach for this calculator
Use it when traffic is the thing you cannot change. If your store gets 60,000 visitors a month and you can run a test for two weeks, this tells you whether a realistic lift is even detectable in that time. If the answer is a 20% MDE, small tweaks are off the table and you should test changes bold enough to clear that bar.
Frequently asked questions
More free CRO tools
Sample size calculator
How many visitors per variant you need to detect a given lift at your chosen confidence and power.
OpenTest duration calculator
How many days a test should run, given your traffic, baseline rate, and the lift you want to detect.
OpenSignificance calculator
Enter visitors and conversions per variant to get the uplift, p-value, and a significance verdict.
OpenConversion rate calculator
Visitors, conversions, and revenue in; conversion rate, average order value, and revenue per visitor out.
OpenRevenue per visitor calculator
Revenue per visitor from revenue and traffic, plus how a lift in RPV flows through to total revenue.
OpenStop guessing what to test
A calculator tells you whether a test is worth running. Spar finds the tests worth running: it audits your storefront, ranks the gaps by revenue impact, and drafts the variations. Start a 14-day free trial.