Sample Size Requirements for Estimating p with margin of error d

To calculate a 95% confidence interval for p with a margin of error no greater than d, use the formula:
n = (1.962)(p)(q) / d2

If no reasonable estimate for p is available, let p = .5, to ensure an adequate sample size.

When sampling from a finite population without replacement, or when using more complex random sampling methods (e.g., cluster sampling), use EpiTable | Sample | Sample Size | Single Proportion to automate calculations.