Calculate confidence intervals for sample means and proportions. Get margin of error, standard error, and confidence bounds for your data.
Calculate confidence intervals for means (with known standard deviation) or proportions.
Get our free PDF with confidence interval formulas, interpretation guidelines, and practical examples.
A confidence interval gives you a range of values within which the true population parameter is likely to fall. The confidence level represents the probability that the interval contains the true parameter.
For Means (with known σ):
CI = x̄ ± Z(α/2) × (σ/√n)
For Proportions:
CI = p̂ ± Z(α/2) × √(p̂(1-p̂)/n)
A 95% confidence interval means that if you repeated your sampling many times, approximately 95% of the calculated intervals would contain the true population parameter.