Confidence Interval Calculator
A confidence interval gives a range that likely contains the true population mean. This calculator uses x̄ ± z*·(s/√n), reporting the critical value, the standard error and the margin of error for a 90%, 95% or 99% level.
Frequently asked questions
What does a 95% confidence interval mean?
If the sampling were repeated many times, about 95% of the intervals built this way would contain the true mean.
What is the margin of error?
It is the half-width of the interval, z*·(s/√n) — how far the bounds sit from the sample mean.
Why does a larger sample give a narrower interval?
The standard error shrinks as √n grows, so more data produces a tighter, more precise interval.