Confidence Interval Calculator
Calculate a confidence interval for a mean (from raw data or summary statistics) or for a proportion, at 90%, 95% or 99% confidence.
How to use
- 1Choose mean or proportion.
- 2Enter the data (or mean, SD and sample size), or successes and trials for a proportion.
How it's calculated
Mean: x̄ ± t* × s ÷ √n (t distribution with n − 1 df). Proportion: p̂ ± z* × √(p̂(1 − p̂) ÷ n) (Wald), plus the Wilson interval for small samples.
Frequently asked questions
What does a 95% confidence interval mean?
If you repeated the study many times, about 95% of intervals built this way would contain the true value.
Why use t instead of z for means?
Because the population standard deviation is estimated from the sample. For large samples t and z are almost identical.
Why show the Wilson interval?
The simple (Wald) interval is inaccurate for small samples or proportions near 0% or 100%; Wilson performs much better.