Confidence Interval Calculator

Confidence Interval Calculator — Calculate CI for Mean & Proportion

The Confidence Interval Calculator is a free statistical tool that computes the confidence interval for a sample mean or proportion, helping you understand the precision of your estimates. Researchers, data scientists, marketers running A/B tests, medical professionals, and students rely on confidence intervals to express how much uncertainty exists around a measurement. With just a few inputs, this calculator returns the lower and upper bounds of the interval at your chosen confidence level (commonly 90%, 95%, or 99%).

What Is a Confidence Interval?

A confidence interval is a range of values that is likely to contain the true population parameter. A 95% confidence interval, for example, means that if you repeated your sampling many times, 95% of the calculated intervals would contain the true value. It does not mean there is a 95% probability the true value falls in any specific interval — a common misinterpretation. Confidence intervals are far more informative than a single point estimate because they communicate both the estimate and its uncertainty.

How to Use the Calculator

Enter your sample size, sample mean (or sample proportion), and standard deviation. Choose your desired confidence level. The tool computes the standard error, looks up the appropriate critical value (z for large samples, t for small samples), and returns the margin of error and the interval bounds. Results are displayed instantly with the formula breakdown.

When to Use Confidence Intervals

Use confidence intervals whenever you want to generalize from a sample to a population: estimating average customer satisfaction from a survey, measuring a campaign's true conversion rate from an A/B test, or reporting clinical trial results. They are required in academic research and industry reports because they convey the statistical strength of your findings far better than p-values alone.

Tamer Baghdadi

CEO / Co-Founder