Confidence Interval Calculator
Estimate the range where the true population mean likely lies, based on your sample mean, standard deviation, sample size, and chosen confidence level (z-interval).
Inputs explained
- Sample mean — The average of your sample data.
- Standard deviation — Standard deviation of the data. Must be positive.
- Sample size (n) — Number of observations in the sample. Must be a whole number.
- Confidence level — How confident you want to be: 90%, 95%, or 99%. Higher confidence gives a wider interval.
How to use it
- Fill in the fields above — each input has a hint describing exactly what to enter
- Press Calculate to get your result instantly, right on this page
- Copy the result, save the tool to favorites, or share and embed it anywhere
Understanding the result
Example: With mean 50, SD 10, n = 25, at 95% confidence the interval is 46.08 to 53.92 (margin of error 3.92).
Frequently asked questions
Why does a higher confidence level widen the interval instead of narrowing it? A higher confidence level (e.g. 99% vs 95%) requires a larger critical z-value to be more certain the true mean falls inside the range, which increases the margin of error and widens the interval.
Does this calculator use the t-distribution for small samples? No, it uses a z-interval based on the standard normal distribution, which assumes the population standard deviation is known or the sample size is reasonably large; for small samples with an estimated standard deviation, a t-distribution would normally be more appropriate.
Is the Confidence Interval Calculator free to use? Yes — it is completely free, with no sign-up, no download and no usage limits. You can use it as often as you need.
Is my data private? Yes. The calculation runs entirely in your browser — the values you enter are never sent to our servers, stored or shared with anyone.
What information do I need? Just the following: Sample mean, Standard deviation, Sample size (n), Confidence level. Each field includes a hint explaining exactly what to enter.
Can I use it on my phone? Yes — the tool is fully responsive and works on any device, and the same calculator is available in the EasyWorkCalcs mobile app.
Privacy
Everything you enter into the Confidence Interval Calculator is processed entirely in your browser. Nothing is uploaded, stored or shared — your data stays on your device.