Sample Size Calculator

For populations over 10,000, the required sample size barely changes because the finite population correction factor approaches 1. This is why national polls of 1,000–2,000 people can reliably represent hundreds of millions — what matters most is sample size, not the fraction of the population sampled.

Margin of error is the maximum expected difference between your sample result and the true population value. A 5% margin of error means your result could be up to 5 percentage points above or below the true value. Halving the margin of error quadruples the required sample size.

Use p = 0.5 (50%) when you have no prior information. This is the most conservative assumption — it maximises the required sample size, ensuring you have enough data regardless of the true proportion. Using p = 0.5 is standard practice for planning surveys.