Free tool

Email A/B Test Calculator

Most A/B-test calculators are built for websites and quietly mislead on email — they ignore that click-to-open is measured among openers, not everyone you sent to. This one is email-native: it sizes a test before you send, gives a clear ship / iterate / kill verdict after, and shows the sweet spot between your list size, baseline and the effect you can actually detect.

Nothing leaves your browser — every calculation runs locally.

Which metric are you testing?

This sets the denominator. Getting it wrong is the single most common email-testing mistake.

Click-to-open is measured only among people who opened. The denominator is openers, not sends — so you must send enough emails that opens × split gives the required number of openers. This tool does that funnel maths for you.

Plan: how big does the test need to be?

Enter what you expect today and the smallest uplift worth acting on.

Baseline rate (current CTO)

Minimum detectable effect (MDE)

15.0% → 16.50%

Confidence

Power

Open rate (openers → sends)

Your available list / segment size (optional — checks feasibility)

Two-proportion z-test · two-sided · normal approximation. A planning aid, not a substitute for judgement — run full weeks, don't peek, and remember open rate is distorted by Apple Mail Privacy Protection.

Disclaimer: this calculator is provided for general guidance only. I accept no liability for any decisions made on the basis of the results — always validate against your own data and context.