Sample Size Calculator
Determine the required sample size for surveys and research studies based on confidence level and margin of error.
Sample Size Needed
Frequently asked questions
How is sample size calculated?
Sample size is calculated using the Z-score corresponding to your confidence level, expected proportion variance, and desired margin of error, adjusted for finite population size.
How large a sample do I need?
It depends on your confidence level, margin of error and expected proportion. For a large population at 95% confidence and a 5% margin, roughly 384 responses suffice; tightening to a 3% margin needs about 1,067, and 1% needs around 9,600. Precision is expensive because required sample grows with the square of the inverse margin.
Why does population size barely matter?
Beyond about 20,000 the finite population correction becomes negligible, so polling a country of 5 million and one of 300 million needs almost the same sample. What matters is the absolute sample size and whether it is genuinely random - which is why a well-drawn sample of 1,000 beats a self-selected sample of 100,000.
What proportion should I assume if I have no prior data?
Use 50%. It maximises the variance p(1-p) and therefore gives the most conservative - largest - required sample, so you cannot end up under-powered. If a pilot study suggests the true proportion is far from 50%, using that estimate reduces the sample you need.