Put a confidence interval around a sample mean so you can express how uncertain your estimate is, rather than reporting a single number as if it were exact.
How to use it
Enter your sample mean, its standard deviation, the number of observations, and the confidence level you want (95% is standard). The interval uses the t-distribution, which is correct for realistic sample sizes.
How to read the result
The interval is the range within which the true population mean is likely to sit. A 95% interval means that if you repeated the study many times, about 95% of the intervals you calculated would contain the true mean. Wider intervals mean more uncertainty.
Rather have it done for you?
Send us your data and a qualified statistician will run the analysis and write it up. Fixed price, by email.
Frequently asked questions
What does a 95% confidence interval actually mean?
It means the method captures the true population value about 95% of the time over repeated sampling. In practice, treat it as a plausible range for the true mean, given your data.
Why use the t-distribution instead of z?
Because you are estimating the standard deviation from your sample rather than knowing it exactly. The t-distribution accounts for that extra uncertainty and is correct for any realistic sample size.
How do I make my interval narrower?
Collect a larger sample or reduce variability in your measurements. Lowering the confidence level narrows the interval too, but at the cost of being less certain.