Statistics & Probability

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99% Confidence Interval Calculator.

Calculate a normal-approximation confidence interval using z = 2.576.

On-device calculationNo signup
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Set your values

Results update as you type.

lower: 45.8784

lower

45.878400
upper: 54.1216

upper

54.121600
marginOfError: 4.1216

marginOfError

4.121600

Results update automatically as you type.

Use Cases

Estimate population mean with high confidence

Use this calculator when you need a very reliable estimate of the population mean from sample data, such as in scientific research or quality control.

Example: A researcher wants to estimate the average height of a species with 99% confidence.

Determine margin of error for surveys

When reporting survey results, a 99% confidence interval provides a conservative margin of error, useful for critical decisions.

Example: A pollster calculates the 99% confidence interval for a candidate's approval rating.

Frequently Asked Questions

What is a 99% confidence interval?
A 99% confidence interval is a range of values that you can be 99% confident contains the true population mean. It is wider than a 95% interval because it provides a higher level of certainty.
How is the 99% confidence interval calculated?
The calculator uses the formula: sample mean ± (z-score for 99% confidence) * (standard deviation / sqrt(sample size)). The z-score for 99% confidence is approximately 2.576.
What inputs do I need?
You need to provide the sample mean, the standard deviation of your sample, and the sample size (number of observations). All fields are required.

Tips & Common Mistakes

Tips

  • Ensure your sample is randomly selected to make the confidence interval valid.
  • The larger the sample size, the narrower the confidence interval, providing a more precise estimate.
  • Use the standard deviation of the sample, not the standard error, as the input.
  • Remember that a 99% confidence interval is wider than a 95% interval, reflecting higher certainty.

Common Mistakes to Avoid

  • Using the sample standard deviation instead of the population standard deviation when it is known.
  • Forgetting to take the square root of the sample size in the denominator.
  • Interpreting the interval as containing 99% of the data, rather than the true mean.

Last updated: August 12, 2026