The Excel CONFIDENCE.T function returns the half-width of a confidence interval for a population mean using the Student's t-distribution — the right choice when the population standard deviation is unknown and estimated from a sample.
Syntax
| Argument | Description | |
|---|---|---|
alpha | Required | The significance level. Use 0.05 for a 95% confidence interval. |
standard_dev | Required | The sample standard deviation. |
size | Required | The sample size (must be greater than 1). |
How to use it
CONFIDENCE.T mirrors CONFIDENCE.NORM but uses the t-distribution, which has fatter tails for small samples — producing a slightly wider, more honest interval when σ is estimated rather than known.
Build the interval as mean ± CONFIDENCE.T(...). As the sample grows, the t-distribution converges to the normal, so CONFIDENCE.T and CONFIDENCE.NORM give nearly identical results for large samples.
Rule of thumb: use CONFIDENCE.T whenever you computed the standard deviation from the sample itself (the usual case). Reserve CONFIDENCE.NORM for the rarer situation where the true population σ is genuinely known.
Try it: interactive demo
Pick a CONFIDENCE.T example to see the formula and its result.
Practice workbook
Frequently asked questions
How is CONFIDENCE.T different from CONFIDENCE.NORM?
What does CONFIDENCE.T return?
Why must the sample size be greater than 1?
size - 1 degrees of freedom. A sample size of 1 leaves zero degrees of freedom, so the function returns an error.When do CONFIDENCE.T and CONFIDENCE.NORM agree?
Master functions like this in one day
This page covers one function. Our Excel Formulas and Functions class covers the 30 that matter most — live, hands-on, taught by professionals in Dallas–Fort Worth, Houston, Austin, Oklahoma City, Denver, or online.
See the Formulas & Functions Class