How to use the Confidence Interval Calculator
The confidence interval calculator gives a range likely to contain the true mean.
Open the Confidence Interval Calculator →
What you need to enter
- Sample mean
- Standard deviation
- Sample size
- Confidence level
The formula
Interval = mean ± z × s ÷ √n.
Worked example
Using the default values in the calculator:
- Sample mean: 50
- Standard deviation: 8
- Sample size: 40
- Confidence level: 90%
| Confidence interval | 47.9192 to 52.0808 |
| Margin of error | 2.0808 |
| Standard error | 1.2649 |
Tips for better results
- Larger samples give narrower intervals.
- The tool uses the normal approximation.
Common mistakes to avoid
- Reading the confidence level as the probability for this one interval.
- Using small samples with z.