Confidence Interval Calculator
Find the range that likely contains the true mean or proportion, from your sample.
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Confidence interval
- Margin of error
- Critical value (z)
- Standard error
- Note
- Work
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How to Calculate a Confidence Interval
Mean: x̄ ± z × s ÷ √n Proportion: p̂ ± z × √(p̂(1 − p̂) ÷ n)
z is the critical value for your confidence level: 1.645 for 90%, 1.960 for 95%, 2.576 for 99%.
Worked Example
Tips
A 95% interval means that if you repeated the sampling many times, about 95% of the intervals would contain the true value. For small samples of means, use the t distribution; this page uses z and says so when n is under 30.