Sample Size Calculator
Tells you how many completed responses you need for results you can rely on. If your population is small — a team, a customer list, a class — enter it and the requirement drops.
Free, no signup, and everything is calculated in your browser.
Responses needed for ±5% at 95% confidence.
How sample size is calculated
n₀ = z² × p(1−p) ÷ e², where z is the z-score for your confidence level, p is the expected proportion, and e is the margin of error as a decimal.
p defaults to 0.5 because that maximises p(1−p) and therefore gives the largest, safest sample. Use a different value only if you have a solid prior estimate.
When you supply a population size N, the finite population correction applies: n = n₀ ÷ (1 + (n₀ − 1)/N). For a 500-person company this often halves what you need.
Questions people ask
How many responses do I need for a survey?
For ±5% at 95% confidence with no population limit, 385. That's the number behind most "about 400 responses" advice. Tighten the margin to ±3% and it jumps to 1,068.
Is this the number of invitations or responses?
Completed responses. To work out invitations, divide by your expected response rate — at 25%, needing 385 responses means inviting roughly 1,540 people.
How many responses do I need for a 5% margin of error?
About 385 at 95% confidence, assuming a large population. If you are surveying a group of 500 people, the finite population correction brings that down to roughly 218, and for a team of 50 you need 45 — nearly everyone.
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