Confidence Interval Calculator
Calculate a confidence interval around a sample mean.
Calculate
Enter your values below.
Result
Lower bound
0
Upper bound
0
Margin of error
0
Formula used
CI = mean ± z × (std dev ÷ √n)
Example: mean 100, std dev 15, n=30, 95% confidence → interval ≈ [94.63, 105.37].
How to use this calculator
- Enter Sample mean, Standard deviation, Sample size (n).
- Choose Confidence level.
- Press Calculate to see your result instantly.
FAQ
Frequently asked questions
What does a confidence interval tell you?
A range that is likely to contain the true population mean, at the stated confidence level.
Why does a higher confidence level widen the interval?
Being more certain of capturing the true mean requires a wider net, using a larger z critical value.
What z-values are used for each level?
1.645 for 90%, 1.96 for 95%, and 2.576 for 99% (two-tailed critical values).
Does a larger sample size narrow the interval?
Yes — larger n reduces the margin of error, since it's divided by √n.