Confidence Interval Calculator
Compute the confidence interval for a mean given the sample standard deviation, sample size, and confidence level.
Inputs
Confidence interval
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Margin of error
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Standard error of mean
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Z-score used
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Interval width
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About Confidence Interval Calculator
How to interpret
A 95% CI means: if you repeat the study many times, 95% of the intervals you construct will contain the true population mean. It does NOT mean "95% chance the true mean is in this interval".
Narrower intervals
Shrink margin of error by: bigger sample (margin scales by 1/√n), lower confidence (90 vs 99), or smaller standard deviation in your data. Sample size is the only one you control.
When to use t instead of z
For small samples (n < 30) or unknown population SD, use the t-distribution. For large n (≥30), t and z are virtually identical. This calculator uses z — fine for most practical n.