📏Confidence Interval Calculator

Calculate a confidence interval for a mean or proportion, with the margin of error and critical value shown step by step.

Sample mean (x̄)

Sample proportion (p̂)

Sample size (n)

Standard deviation

Confidence level

Custom confidence level (%)

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Lower bound

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Upper bound

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Margin of error

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Critical value

Step-by-Step Working

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Free Confidence Interval Calculator

Need a confidence interval for a mean or proportion? Our free Confidence Interval Calculator computes the margin of error and interval bounds instantly, choosing the right method automatically — a z-interval when the population standard deviation is known, or a t-interval when it isn't.

Enter your sample mean or proportion, sample size, and standard deviation, pick a confidence level (90%, 95%, 99%, or a custom value), and get the full interval along with the critical value and standard error used to build it. Every step is shown, from the critical value lookup to the final margin-of-error formula, so you can follow — or verify — the calculation by hand.

Built for statistics students and researchers who need accurate, transparent results without a textbook z-table or t-table. Free, with no sign-up and no limits. See the About page or the FAQ for more.