Find the sample size needed to estimate a proportion at a chosen confidence and margin of error. Pick the confidence and enter the margin.
Usage Tip
Halving the margin of error quadruples the required sample size — precision gets expensive fast.
z from the confidence level, E the margin of error
A proportion of 50 percent is the most conservative (largest) sample requirement.
The expected proportion defaults to 50 percent (worst case).
The result is rounded up to a whole number of responses.
This calculator finds how many responses are needed to estimate a proportion within a chosen margin of error and confidence level. The standard formula multiplies the squared z value for your confidence level by the expected proportion times one minus that proportion, then divides by the squared margin of error.
If you have no idea what proportion to expect, use 50 percent, which gives the largest and therefore safest sample size. Plan for non-response by dividing the answer by the share of people you expect to reply. For small populations, the required sample can be reduced with a finite population correction.