# A/B test sample size calculator

> To know how many visitors your A/B test needs, enter your current conversion rate and the smallest lift worth detecting: the calculator gives you visitors per variant at 95% confidence and 80% power, the usual values.

- Source: https://bixlow.com/en/tools/sample-size-calculator
- Language: en
- Updated: 2026-11-02
- Publisher: Bixlow

## How to read the result

The number is per variant. If your test has the original and one variant, you need twice that in total. With three versions, three times.

What moves the result most is the minimum lift. Detecting +20% needs about four times less traffic than detecting +10%. That is why, if your visits are limited, it pays to test big changes rather than colour tweaks.


## Some reference values

Visitors per variant, at 95% confidence and 80% power.

| Current conversion | Lift to detect +10% | Lift to detect +20% | Lift to detect +30% |
| --- | --- | --- | --- |
| 1% | 163,100 | 42,700 | 19,800 |
| 2% | 80,700 | 21,100 | 9,800 |
| 5% | 31,200 | 8,200 | 3,800 |
| 10% | 14,800 | 3,800 | 1,800 |

_Rounded to the nearest hundred. Computed with the same formula as the calculator._


## What confidence and power mean

Confidence is how sure you want to be of not accepting a lift that does not exist. Power is the chance of finding the lift if it really exists. At 95% and 80%, out of every five real lifts you test you will find four, and you will accept a false one around one time in twenty.


> **A warning.** Do not check the result every day and stop when you like what you see. It is the fastest way to declare winners that are not. Decide beforehand how much traffic you need and wait until you have it.


## How Bixlow does it

This calculator uses the classic two-proportion formula, the same as almost every calculator. Bixlow reads results with Bayesian statistics and does not name a winner until there is minimum sample and duration and at least a 95% probability that one version is best. The order of magnitude of traffic you need is the same, so this number works for planning.


## FAQ

### What if I get a huge number?

Test bigger changes, ones with a real chance of moving conversion 20% or more, or measure a more frequent conversion, such as add to cart instead of purchase.

### Does it count visitors or sessions?

Unique visitors who see the test page. If one person comes three times, they count once.

### Does it work for tests with more than two versions?

Yes, multiply the result by the number of versions, original included, as a first estimate. With three or more, each version also needs a bit more sample, because every variant is compared with the original and those extra comparisons have to be corrected for. The duration calculator already accounts for it.

## Further reading

- [A/B test duration calculator](https://bixlow.com/en/tools/ab-test-duration-calculator)
- [What is sample size](https://bixlow.com/en/glossary/sample-size)
- [What is the minimum detectable effect](https://bixlow.com/en/glossary/minimum-detectable-effect)
- [Bixlow's statistics](https://bixlow.com/en/platform/statistics)