Confidence Interval Calculator

How confident you want to be that the interval contains the true value. Higher levels give wider intervals.

Enter a percentage, e.g. 95 for the conventional 95% interval.

The number of observations in your sample. Larger samples give narrower intervals — the width shrinks roughly with the square root of n.

The average of your sample — the centre point that the confidence interval is built around.

The spread of the data: use the sample standard deviation for the t and χ² intervals, or the known population σ for the z interval.

How many of the n observations had the outcome of interest — e.g. 42 of 200 respondents said yes. The sample proportion is successes divided by n.