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Confidence Interval Calculator
Build confidence intervals for means, proportions, and differences between groups at 80%, 90%, 95%, or 99% confidence levels.
Interval type
Confidence interval for a single mean using z* s / sqrt(n).
Inputs
Critical value method
confidence interval calculator95% confidence interval for mean or proportion
Interval summary
Critical value (z* / t*)
1.96
Degrees of freedom (t)
29
Point estimate
10
Margin of error
0.715691
Confidence interval
(9.284309, 10.715691)
How to use this tool
- Choose the type of confidence interval you want to build (mean, proportion, or difference).
- Enter the relevant sample statistics and select a confidence level such as 95%.
- Read off the point estimate, margin of error, and the lower and upper bounds.
Confidence interval calculator for exams
- Quickly check 95% confidence intervals for means and proportions against your working.
- Experiment with different confidence levels to see how the margin of error changes.
- Use the difference in means or proportions to compare two groups with a single interval.
What this CI calculator assumes
- Uses z critical values by default, with optional t critical values for small-sample mean intervals.
- Assumes reasonably large samples or approximate normality so normal and t approximations are valid.
- Intervals are two-sided and symmetric around the point estimate.
FAQ
- Does this use z or t for the confidence interval?
- You can choose. For large samples, use z. For small samples when estimating a mean, switch to t and the calculator will use an approximate t critical value based on the degrees of freedom.
- Can I enter percentages instead of proportions?
- Enter proportions as decimals between 0 and 1 (for example, 0.4 instead of 40%). This keeps the formulas consistent and reduces confusion.
- What does a 95% confidence interval mean in plain language?
- Roughly speaking, if you repeated the sampling process many times, about 95% of the constructed intervals would contain the true parameter.
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