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Confidence Interval Calculator
Build an interval estimate for a population mean from a sample mean, standard deviation and sample size.
Confidence interval for a mean
Confidence interval: —
Margin of error: —
Standard error: —
Critical z value: —
How to use the Confidence Interval Calculator
- Enter the sample mean, standard deviation and sample size.
- Pick the confidence level you want, such as 95 percent.
- Read the interval, margin of error and standard error in the result panel.
Features
- Confidence levels from 90 to 99.9 percent
- Interval, margin of error and standard error
- Critical z value shown for the level
- Runs locally, no uploads
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Frequently Asked Questions
Common questions about the free online confidence interval calculator.
Enter the sample mean, standard deviation and sample size, pick a confidence level, and the
interval appears instantly.
It means that if you repeated the study many times, about 95 percent of the intervals you
built would contain the true mean.
It is the amount added to and subtracted from the sample mean, equal to the critical z value
times the standard error.
The critical value is 1.96, which leaves 2.5 percent of the normal curve in each tail of the
distribution.
Use a t distribution when the sample is small and the standard deviation is unknown, because
z makes the interval too narrow.
A larger sample lowers the standard error, so the confidence interval becomes narrower and
the estimate becomes more precise.
It is the standard deviation divided by the square root of the sample size, and it measures
how variable the sample mean is.
Raise the confidence level to widen the interval, or collect more data to narrow it without
losing confidence.
No. Every calculation runs locally in your browser, so your numbers never leave your device.
Yes. It is completely free and unlimited, with no account, no sign-up and no usage limits of
any kind.
Last updated: October 10, 2026