A/B Testing Calculator

What Is Statistical Significance?

Statistical significance tells you whether the difference between your control and variation is real or just due to random chance. A 95% confidence level means there is only a 5% probability that the observed difference happened by luck. Without reaching statistical significance, you cannot trust your test results to hold up over time.

Why Sample Size Matters

Running a test with too few visitors leads to unreliable results. Small samples amplify random noise, making it easy to mistake a fluke for a real improvement. Calculating your required sample size before you start ensures your test has enough statistical power to detect the effect you are looking for. Ending a test early because it "looks like a winner" is one of the most common and costly mistakes in A/B testing.

Common A/B Testing Mistakes

• 1.Stopping too early. Calling a test before reaching your required sample size dramatically increases the chance of a false positive.
• 2.Testing too many variations. Each additional variation increases the total sample you need. Focus on one clear hypothesis at a time.
• 3.Ignoring external factors. Seasonality, promotions, and traffic source changes can skew results. Run tests during stable traffic periods when possible.
• 4.Not defining success metrics upfront. Decide what you are measuring before the test starts. Changing your primary metric after the fact introduces bias.
• 5.Testing tiny changes on low-traffic pages. Small effects require enormous sample sizes to detect. Focus on high-impact changes where you have enough traffic to reach significance in a reasonable timeframe.

How This Calculator Works

This calculator uses a standard two-proportion z-test formula. It takes your baseline conversion rate and the minimum improvement you want to detect, then calculates how many visitors each variation needs using a 95% confidence level (z = 1.96) and 80% statistical power (z = 0.84). The estimated duration divides the total sample size across both variations by your daily traffic.