ABFinal

Calculators

Free A/B Testing Calculators

Seven calculators that cover a test from the first estimate to the final call. They run in your browser, update as you type, and show their formulas.

Calculator

Sample size

How many visitors each variant needs to detect the change you care about, at your chosen confidence and power.

Calculator

Significance

Paste visitors and conversions for A and B. Get the p-value, confidence interval for the lift and the Bayesian chance that B beats A.

Calculator

Test duration

Turn the required sample size into days and full weeks, based on your daily traffic and the share of it in the test.

Calculator

MDE

Given your traffic, find the smallest lift (and drop) a test can reliably detect, before you spend weeks on it.

Calculator

SRM checker

Check whether the visitors in each group match the split you planned. A mismatch means something in the setup is broken.

Calculator

Price test

Compare two prices on conversion, revenue per visitor and gross profit per visitor, with the break-even conversion rate for the new price.

Calculator

A/A simulator

Simulate thousands of tests where nothing changed and watch how often a false winner appears, with and without peeking.

Which tool for which moment

Stage Question Tool
Before launch How many visitors do I need? Sample size
Before launch How many days is that? Test duration
Before launch What can I detect with my traffic? MDE
During the test Is the traffic split healthy? SRM checker
After the test Did B win, and by how much? Significance
Price tests Did profit go up? Price test
Learning How often do false winners happen? A/A simulator

Testing posts on X? The X testing hub has a post comparer and a variant planner.

All formulas, reference values and known limitations are on the methodology page.

Common questions

Which calculator should I start with?

Start with the sample size calculator, then the duration calculator to turn that number into days. If the duration is too long, the MDE calculator shows what you can detect with the traffic you have.

Do the calculators work on my phone?

Yes. They're plain HTML and JavaScript with no plugins and work in any current mobile browser, offline too once the page has loaded.

Can I use the results in a report?

Yes. The significance calculator has a "Copy summary" button, and every page explains its method so you can cite it. Linking to the page you used is appreciated but not required.