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Confidence Interval Calculator
Compute confidence intervals for a population mean or proportion at 80-99.9% confidence using the z-distribution.
MATHCompute a confidence interval for either a population mean (with known/estimated standard deviation) or a population proportion. Supports 80%, 90%, 95%, 98%, 99%, and 99.9% confidence levels.
CI for mean: x̄ ± z × (σ/√n). CI for proportion: p̂ ± z × √(p̂(1−p̂)/n). Z-values for 95% CI is 1.96; for 99% is 2.576. Wider CI = more confidence, less precision. Doubling sample size narrows CI by factor of √2.
Disclaimer: Uses normal (z) distribution. For small samples (n < 30) with unknown σ, use t-distribution. For proportions near 0 or 1, Wilson score or Clopper-Pearson is more reliable.
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Frequently Asked Questions
What is the Confidence Interval Calculator for?
Compute a confidence interval for either a population mean (with known/estimated standard deviation) or a population proportion. Supports 80%, 90%, 95%, 98%, 99%, and 99.9% confidence levels.
How do I use the Confidence Interval Calculator?
CI for mean: x̄ ± z × (σ/√n). CI for proportion: p̂ ± z × √(p̂(1−p̂)/n). Z-values for 95% CI is 1.96; for 99% is 2.576. Wider CI = more confidence, less precision. Doubling sample size narrows CI by factor of √2.
How accurate are my confidence interval results?
Uses normal (z) distribution. For small samples (n < 30) with unknown σ, use t-distribution. For proportions near 0 or 1, Wilson score or Clopper-Pearson is more reliable.
Is the Confidence Interval Calculator free to use?
Yes - every calculator on WhichCalc is completely free with no signup, no usage limits, and no tracking on the calculation itself. Results display instantly in your browser and your inputs are never sent to a server. Bookmark the page if you use this calculator regularly.
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