Sample Size Calculator
Free calculator for survey sample size. Enter your population, confidence level, and margin of error to get the required number of completes.
±1%±5% (standard)±10%
Required sample size
385
completes at 95% confidence, ±5% margin of error
How this calculator works
For a large population, the required sample size at a given confidence level (z-score), margin of error (e), and expected proportion (p) is:
n = (z² × p × (1 − p)) / e²For a finite population N, we apply the finite population correction:
n_adj = n / (1 + (n − 1) / N)p = 0.5 is the most conservative default — it produces the largest required sample. Use a different proportion only if you have prior knowledge of the expected response distribution.
Practical guidance
- Always plan for completes, not invites. Account for incidence rate and completion rate. To get 400 completes from a niche audience, you may need to invite 5,000.
- Pad for sub-segments. If you'll report on segments, each segment needs a meaningful sample of its own — not just the overall total.
- Don't over-target margin of error. ±3% sounds rigorous but the cost is ~3× the sample. Most decisions don't need that precision.
Read the full guide: Sample size calculation methodology.