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Math Calculators

Sample Size Calculator

Verified formula Updated Jun 2026 Private — runs on your device

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Verified formula Private

Required sample size

385

For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.

How to use the Sample Size Calculator

The Sample Size Calculator works out your required sample size in an instant. Enter margin of error, proportion and confidence level and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.

  1. Enter the margin of error.
  2. Enter the proportion.
  3. Choose the confidence level.
  4. Read off your required sample size — the calculator updates automatically, with no button to press.

Worked example

For example, with margin of error of 5%, proportion of 0.5 and confidence level of 95% (z = 1.96), the required sample size is 385.

Inputs used
Margin of error 5%
Proportion 0.5
Confidence level 95% (z = 1.96)
Results
Required sample size 385

Results are estimates for educational use, not professional advice.

Frequently asked questions

Use n = z² × p(1 − p) ÷ margin². For a 5% margin at 95% confidence with p = 0.5, you need about 385 responses.

It produces the largest required sample, so your survey is adequately sized even if the true proportion is unknown.

For large populations, not much. This formula assumes a large population; for small populations a finite-population correction reduces the number needed.

Accept a larger margin of error or a lower confidence level. Both reduce the responses required, at the cost of precision or certainty.

Enter the margin of error. Enter the proportion. Choose the confidence level. Read off your required sample size — the calculator updates automatically, with no button to press.

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