# Confidence Interval Calculator

> Confidence interval calculator: build a confidence interval for a mean from sample size, mean, and standard deviation. Free, private, runs in your browser.

- **Canonical URL:** https://boringtoolskit.com/confidence-interval-calculator/
- **Category:** Everyday Calculators
- **Evidence level:** authored-methodology
- **Reviewed:** August 2026
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

CI = mean +/- z* x sd / sqrt(n); z* = 1.96 for 95% confidence.

## Worked example

Mean 100, standard deviation 15, n = 36: standard error 2.5, margin of error +/-4.9, a 95% interval of [95.1, 104.9].

## Sources

- [Calculation methodology and verification standard](/methodology/)

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