Survey Sample Size Calculator
Choose your confidence level and acceptable margin of error, optionally add your population size, and get the minimum number of completed responses you should collect.
The formula
What the result means
The confidence level (via its z-score: 1.645 for 90%, 1.96 for 95%, 2.576 for 99%) expresses how sure you want to be that the true value falls within your margin of error. The expected distribution p is what share you expect to answer a given way — 50% is the safest assumption because it maximizes the required sample.
When you provide a population size, the finite population correction reduces the required sample — surveying 300 people matters more in a population of 1,000 than in one of 1,000,000.
Remember this is the number of completed responses you need, not invitations. Divide by your expected response rate to plan the send.