Sample Size Calculator (Mean / Proportion / t-Test Power)
Online sample size calculator: mean estimation, proportion estimation and two-sample t-test power analysis (exact solution using the noncentral t distribution), with sensitivity analysis.
How many samples are needed to estimate a proportion with 95% confidence and a ±5% margin of error?
In the most conservative case of p = 0.5, about 385 samples are needed: n = 1.96² × 0.25 / 0.05².