Online Tool Store
Conversion Rate Calculator
Conversion Rate Calculator reports the interval, not just the rate — because two rates that look different often are not.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
SEO & Web
Conversion Rate Calculator
Frontend preview — no upload or external service.
Comparison
Variant A: 3.3% CR. Variant B: 3.97% CR. B is 20% higher. Result is 92% statistically significant at 95% confidence.
How the Conversion Rate Calculator works
- Enter visitors and conversions for each variant.
- Read the confidence intervals rather than the point estimates, since two rates that look different often have overlapping intervals.
- If the intervals overlap, keep running. Stopping a test the moment it looks favourable is the most common way to reach a wrong conclusion.
The method
A conversion rate from a sample carries uncertainty that shrinks with sample size.
interval width is proportional to sqrt(p(1-p)/n)
Detecting a small relative lift needs a much larger sample than detecting a large one, which is why underpowered tests report noise as results.
FAQ
When can I stop a test?
When it reaches the sample size you calculated before starting. Stopping as soon as a result looks significant — peeking — inflates false positives substantially, and it is the most common testing error.
Why do the intervals overlap when the rates look different?
Because the difference is small relative to the noise at that sample size. A 20% relative lift on a 3% base rate needs thousands of conversions per variant, not thousands of visitors.
Is a 20% lift meaningful?
It depends entirely on whether it is real. A large relative lift on a small sample is usually noise, and relative lifts look impressive precisely because the base rate is small.
How we compare
| Feature | Online Tool Store | An SEO suite | Hand-written markup |
|---|---|---|---|
| Confidence intervals | ✓ | ✗ | ✓ |
| Warns about peeking | ✓ | ✗ | ✗ |
| No sign-up | ✓ | ✗ | ✗ |
| Nothing uploaded | ✓ | ✗ | ✗ |
Conversion Rate Calculator reports the interval, not just the rate — because two rates that look different often are not.