How to use Sample Size Calculator

Choose mean or proportion precision.
Enter confidence, error and the required variability assumption.
Review the rounded sample count and any population adjustment.

Example: Sample Size Calculator

Sample Size Calculator: Plan at least 385 observations under the stated precision assumptions.

You add

Estimate to plan: proportion Confidence level (%): 95 Desired half-width: 0.05 Planning proportion: 0.5 Planning standard deviation: 10 Finite population size (optional): (blank)

You get

Plan at least 385 observations under the stated precision assumptions. Planned sample size (rounded up): 385 Estimate: proportion Confidence level: 95% Desired half-width: 0.05 Critical z: 1.95996398454 Planning proportion or SD: 0.5 Unrounded infinite-population n₀: 384.145882069 Population size: Not supplied Unrounded final n: 384.145882069 n₀ = z²p(1−p)/E² for a proportion or z²σ²/E² for a mean. With finite population N, n = Nn₀/(N+n₀−1), then round up. This is a normal-approximation precision plan, not a power calculation or guarantee of achieved coverage. It does not include nonresponse, design effects, clustering, estimated-SD t planning or rare-outcome exact intervals. Proportion plans near 0 or 1 may ne Planned sample size (rounded up) | 385 Estimate | proportion Confidence level | 95%

Options

Desired half-width: Enter the margin in the units of the estimate. For a proportion, 0.05 means five percentage points.
Planning assumptions: Use a planning proportion or deviation appropriate to the chosen method. A finite-population adjustment is a separate assumption.

Supported inputs and limits

Precision calculations under the stated normal approximation and sampling assumptions. A finite population adjustment applies only when its assumptions are appropriate. This is not a hypothesis-test power calculator; nonresponse, clustering and selection bias are not inferred.

Where your input is processed

This tool processes your input in this browser. Your text and files are not uploaded to UseFreeTools. Check this tool's limits for anything it may save on your device.

The count is for precision, not test power

The result rounds up a normal-approximation precision calculation. It does not plan power for a hypothesis test or automatically allow for nonresponse, clustering or selection bias. Keep the inputs with the count and account for the collection design separately.

Questions about Sample Size Calculator

What does a proportion of 0.5 mean here?

It supplies the largest variance for the supported proportion calculation when you have no better planning estimate.

Why is the count rounded upward?

A smaller whole count could miss the precision target under the entered model.

Does the result account for nonresponse?

No. Plan participation and sampling design separately.

Project manager: Tony Hines · Content updated 4 October 2026 · Report a problem

Sample Size Calculator

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