Confidence Interval Calculator

Confidence intervals for means, proportions, differences, odds ratios and relative risks.

Confidence interval for

Mean

Successes (events)

Total

Mean 1

SD 1

n 1

Mean 2

SD 2

n 2

Group 1 events

Group 1 total

Group 2 events

Group 2 total

Exposed: events

Exposed: no event

Unexposed: events

Unexposed: no event

Confidence level

Enter your values to see the result.

Results use standard large-sample formulas and are meant to support, not replace, a statistician's review. Check the assumptions behind each method before you report the numbers.

Continue your work

SD, SE & CI Converter — Work backwards from an interval to the SD.
Sample Size Calculator — Plan a sample that gives a narrow enough interval.

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Clear instructions

Find steps, examples and limitations below.

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How to use the Confidence Interval Calculator

Confidence intervals for means, proportions, differences, odds ratios and relative risks.

1 Choose what you need an interval for.
2 Enter the summary figures or counts and pick the confidence level.
3 Read the interval, with alternative methods for comparison.

Example and practical tips

A mean of 120 with SD 15 from 50 people gives a 95% confidence interval of 115.74 to 124.26.

Frequently asked questions

Which interval should I use for a proportion?

The Wilson score interval works well at all sample sizes. The simple Wald interval is unreliable when counts are small or near 0% or 100%.

Why use the t distribution for a mean?

With an estimated SD, the t distribution gives the correct width, especially for samples under about 30.