Online Tool Store
Confidence Interval Calculator
Enter a sample mean, standard deviation, and sample size to get the confidence interval at 90%, 95%, or 99%. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
95% confidence interval
Example shown — replace it with your own sample statistics.
How it works
- Enter the sample mean, standard deviation, and sample size.
- Choose a confidence level.
- The interval is calculated and shown instantly.
The interval is computed as mean ± z × (standard deviation / √n), where z is the critical value for the chosen confidence level (1.645 for 90%, 1.96 for 95%, 2.576 for 99%) — the standard formula for a confidence interval around a sample mean.
FAQ
Is my data uploaded anywhere?
No. The calculation happens entirely in your browser — your sample data is never sent to a server.
What confidence levels are supported?
90%, 95%, and 99% are available, each using the corresponding z-score for the margin of error.
Does it use the z-distribution or t-distribution?
The z-distribution, appropriate for larger samples (n ≥ 30) or a known population standard deviation.
What does the confidence interval actually tell me?
It gives a range that, if you repeated the sampling many times, would contain the true population mean the chosen percentage of the time.
Does sample size affect the interval width?
Yes — since the margin of error divides the standard deviation by √n, a larger sample produces a narrower interval for the same standard deviation and confidence level.
How we compare
| Feature | Online Tool Store | Spreadsheet formula | Statistics software |
|---|---|---|---|
| No file upload — runs in your browser | ✓ | ✓ | Desktop install required |
| Free, no setup | ✓ | ✓ | Often paid (SPSS, Minitab) |
Statistics software is built for full analyses. Confidence Interval Calculator is for the quick, one-off check — three numbers in, an interval out.