Calculate the minimum sample size needed for a survey or study, based on your desired confidence level, margin of error and expected population proportion.
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This calculator uses the standard survey sample size formula: n = (z² × p × (1 − p)) ÷ e², where z is the critical value for your confidence level, p is the expected proportion (50% is the most conservative, producing the largest sample size), and e is your margin of error. If you enter a population size, a finite population correction is applied to reduce the required sample size for smaller populations.
When the true proportion is unknown, 50% produces the largest possible required sample size, making it the safest, most conservative default. If you have a reasonable estimate of the true proportion from prior data, using it can reduce your required sample size.
No, leave it blank for a large or unknown population. Only enter it if you're sampling from a small, well-defined population (like all employees at a single company), since it can meaningfully reduce the required sample size.
5% is a common standard for general surveys. Tighter margins (like 1-3%) require substantially larger sample sizes.
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