How many participants does your study need?

Plan your sample size, understand the assumptions, and take a clear calculation into your research protocol.

Find the right procedure I know my method — open the calculator

Free to use. No account. English and Arabic.

Planning a blood-pressure study

A worked example: compare mean systolic blood pressure in two independent groups.

Difference to detect: 5 mmHg
Standard deviation: 10 mmHg
Statistical power: 80%
Significance level: 5% · two-sided

These assumptions require 64 participants per group — 128 in total.

Independent two-sample t-test, equal group sizes and a common standard deviation. No allowance for dropout. These are example inputs, not clinical recommendations.

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A sample size you can explain.

Start with your research question. Then check how the answer changes when your assumptions change.

Explore Sample Size

Choose a suitable method: Use the guide to connect your outcome and study design to a statistical procedure.
See what drives the result: Review effect size, precision, power and expected dropout. Use sensitivity curves to compare scenarios.
Keep the reasoning with the number: Export the result with its inputs and calculation details for your research notes.