Confidence Interval Calculator
Calculate a confidence interval for a sample mean.
About this tool
A confidence interval gives a range that likely contains the true population mean, based on your sample's mean, standard deviation, size and a chosen confidence level \u2014 using the standard z-based formula for large samples.
Key Features
- 90%, 95% or 99% confidence levels
- Margin of error and full interval
- Standard z-based formula (large-sample approximation)
How to Use
- Enter your sample's mean and standard deviation.
- Enter the sample size.
- Choose a confidence level.
- Click Calculate Interval.
Benefits
- Check statistics homework
- Report survey or experiment results with a proper interval
- Understand how sample size affects precision
Frequently Asked Questions
It means that if you repeated this sampling process many times and built an interval each time the same way, about 95% of those intervals would contain the true population mean \u2014 it's a statement about the method's reliability, not a 95% probability that this specific interval contains the true value.
The margin of error is proportional to 1/\u221an \u2014 as sample size grows, your estimate of the mean becomes more precise, so the interval needed to be 90/95/99% confident gets narrower.