Frequently Asked Questions
Everything you need to know about the Confidence Interval Calculator.
What is a confidence interval?
A range of values, calculated from sample data, that's likely to contain the true population parameter at a given confidence level.
When should I use a z-interval vs a t-interval?
Use a z-interval when the population standard deviation is known; use a t-interval when you only know the sample standard deviation, which is the more common real-world case.
How is a confidence interval calculated?
As the sample statistic ยฑ the margin of error, where the margin of error is the critical value times the standard error.
What confidence levels can I choose?
90%, 95%, and 99% are built in, plus a custom percentage.
Can I calculate a confidence interval for a proportion?
Yes, switch to proportion mode and enter your observed proportion and sample size.
What is the critical value?
The z* or t* value corresponding to your chosen confidence level and (for t) degrees of freedom; it's shown alongside your result.
What sample size do I need?
Any n โฅ 2 works for a t-interval; larger samples generally give a narrower, more precise interval.
Does it show the calculation steps?
Yes, the standard error, critical value, and margin-of-error formula are all shown.
Is my data stored?
No, all calculations run client-side in your browser.
Is it free?
Yes, completely free with no sign-up or limits.