Confidence Interval Calculator
Enter your sample statistics and confidence level.
How this calculator works
Confidence interval = mean +/- z x (standard deviation / sqrt(n)). The standard error, s / sqrt(n), shrinks as the sample grows, which narrows the interval. This uses the normal z value, which suits samples of about 30 or more.
Frequently asked questions
What does a 95% confidence interval mean?
It is a range built so that, over many samples, about 95% of such ranges contain the true mean.
When should I use a t value instead?
For small samples, under about 30, with an unknown population deviation.
How can I make the interval narrower?
Increase the sample size or accept a lower confidence level.
