How to use the Confidence Interval Calculator
- Choose a mean or a proportion.
- Enter your sample results and pick a confidence level.
- Read the interval and margin of error.
What does this tool do?
A 95% confidence interval is built so that, if you repeated the study many times, about 95% of intervals would contain the true population value. Wider intervals mean more uncertainty.
For means, the calculator uses the t-distribution when the standard deviation comes from your sample — the usual case — and the normal distribution only when the population σ is known. For proportions it shows the recommended Wilson interval alongside the simple Wald interval.
Why use it?
- Uses t automatically for sample standard deviations.
- Wilson interval for proportions.
- Shows the critical value and standard error.
The formula
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Frequently asked questions
Should I use 95% or 99%?
95% is the common default. 99% gives a wider, more cautious interval; 90% a narrower one.
Last reviewed by the M2Toolkit team.