About the Confidence Interval Calculator
Compute z-intervals, t-intervals, and proportion intervals with the full working shown.
What it is
A confidence interval estimates a range likely to contain a true population parameter, built from a sample statistic plus or minus a margin of error. This tool handles the three most common cases โ a mean with known population standard deviation (z-interval), a mean with only the sample standard deviation (t-interval), and a proportion โ computing accurate critical values numerically rather than from a rounded lookup table.
Features
Three modes
z-interval, t-interval, and proportion interval, selected with a single toggle.
Accurate critical values
Computed via the regularized incomplete beta function, not a limited lookup table.
Custom confidence levels
90%, 95%, 99%, or any custom percentage in between.
Step-by-step working
Standard error, critical value, and margin-of-error substitution shown for every result.