Confidence Interval Calculator
Compute a confidence interval directly from summary statistics — no raw data needed. For a mean, enter the sample mean, standard deviation and n, and choose the t method (population SD unknown — the usual case) or the z method (population SD known). For a proportion, enter the number of successes and the sample size; the calculator uses the Wald normal approximation and clamps the interval to [0, 1]. You get the interval bounds plus everything needed to show your work: standard error, critical value (t* or z*) and margin of error. APA 7 now expects confidence intervals alongside point estimates, so the AI Report button formats the result into a publication-ready sentence.
AI Report
Let AI interpret your results: a downloadable Word document in APA 7 / business report format.
Frequently asked questions
How do I report a confidence interval in APA 7 format?
Give the confidence level once, then the bounds in square brackets right after the estimate: "M = 50.0, 95% CI [46.70, 53.30]". APA 7 explicitly encourages reporting confidence intervals for every point estimate. The copy button gives you this exact string, and the AI Report expands it into a full results paragraph.
Should I use the t or the z method for a mean?
Use t (the default) whenever the population standard deviation is unknown and you estimated it from the sample — that is almost every real study. The z method is only correct when σ is genuinely known in advance or the sample is very large, where the two methods converge anyway.
What does "95% confident" actually mean?
It refers to the procedure, not this single interval: if you repeated the study many times and built an interval each time, about 95% of those intervals would contain the true population value. It is not a 95% probability that the true value lies inside this particular interval — a common misreading.