Free tool · no login
Sample size calculator for ad and landing page tests
Enter your current conversion rate and the smallest lift worth detecting to get the visitors each version needs, and add daily traffic and cost per click to see how long and how much the test will take.
Sample size
Enter your numbers to see the result.
Free tool by Telehealth Media · telehealthmedia.com
How it works
The calculator uses the standard formula for a two-sided test of two proportions.
- Rate 1 = current conversion rate; rate 2 = rate 1 x (1 + lift)
- Visitors per version = [z for significance x root(2 x p x (1 - p)) + z for power x root(rate 1 x (1 - rate 1) + rate 2 x (1 - rate 2))] squared / (rate 2 - rate 1) squared, rounded up, where p is the average of the two rates
Significance defaults to 0.05 and power to 0.8, the usual choices. Smaller lifts and lower conversion rates need many more visitors.
Worked example
A landing page converts at 3% and the smallest lift worth finding is 20% (to 3.6%).
| Step | Result |
|---|---|
| Visitors per version | 13,914 |
| Visitors in total | 27,828 |
These numbers illustrate the formula; enter your own.
Telehealth notes
- Patient conversion rates are low, so tests on paying patients need a lot of traffic. Testing intake starts first is faster.
- Decide the sample size before starting, and do not stop early when the result looks good.
Questions
How many visitors do I need for an A/B test?
It depends on your conversion rate and the smallest lift you want to detect. This calculator gives the number for each version.
What are significance and power?
Significance is how often you accept a false win (0.05 means 5 times in 100). Power is how often you catch a real lift of the size you chose.
Are my numbers stored?
No. The calculation runs in your browser and nothing is sent to us.