Confidence Interval Calculator
Estimate a two-sided mean interval from sample mean, standard deviation, size and a preset level.
What would you like to do next?
Turn sample summaries into a visible interval
Enter a sample mean, sample standard deviation, whole-number sample size and a confidence level of 90, 95 or 99. The calculator finds the standard error, applies the matching two-sided z critical value and reports lower and upper bounds plus margin of error. Keeping each intermediate quantity visible makes the simplifying model easier to review.
Match the interval method to the study design
This quick interval treats the observations as independent and uses a z critical value with the entered standard deviation. Small samples often call for a t interval, while clustering, weighting, finite populations, skewed data and complex sampling need design-aware methods. Confidence level describes repeated-procedure coverage, not the probability that one fixed interval contains the parameter.
Frequently asked questions
Which confidence levels are supported?
Enter 90, 95 or 99 for the preset two-sided z critical values.
What is the standard error?
The entered standard deviation is divided by the square root of sample size.
Is this always the right interval?
No. Small or complex samples may require t-based or design-aware analysis.