Confidence Interval Calculator
For a mean or a proportion, with the range you can actually defend.
- Free
- Runs in your browser
- No sign-up
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Enter the sample mean, standard deviation and size.
Runs entirely in your browser. Nothing is uploaded.
How it works
What a 95% confidence interval means
It does not mean there is a 95% chance the true value is in this particular range. It means the method produces an interval that captures the true value 95% of the time across repeated samples. In practice: report the interval, not just the point estimate. “12% ± 3” is honest; “12%” alone pretends to a precision you don't have.
Why the mean uses t, not z
When you estimate the standard deviation from the sample (you almost always do), the t-distribution accounts for that extra uncertainty: its intervals are slightly wider for small samples and converge to the normal (z) result as n grows. Using z on a small sample understates the interval, which is the common mistake this avoids.
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