Calculate confidence intervals for a mean or a proportion — choose your confidence level (90%, 95%, 99% or custom), and see the margin of error and lower and upper bounds.
Enter your values on the left, then press Calculate.
A 95% confidence interval gives you the range that the true population value is likely to fall within, with 95% confidence. Wider intervals mean more uncertainty; narrower intervals mean more precision — but require larger sample sizes.
For continuous data (heights, weights, cycle times) use the mean mode. For pass/fail or yes/no data (defective rate, conversion rate, customer satisfaction %) use the proportion mode — the maths is different.
Going from 95% to 99% confidence widens your interval by about 30% — meaning more uncertainty for the same data. To narrow the interval, the only lever is sample size: doubling n cuts the margin of error by about 30%.