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Updated 2026-03-07 UTC
Sample Size for a Proportion — Margin of Error Formula — GetCalcMaster
Compute sample size needed for estimating a proportion with a target margin of error: n ≈ z²·p(1−p)/E². Includes practical notes and examples.
To estimate a proportion p with a desired margin of error E at a given confidence level, a common approximation is n ≈ z²·p(1−p)/E².
Important: Educational. For strict guarantees, use an exact CI-based planning method.
What this calculator is
The Statistics Calculator is an interactive tool inside GetCalcMaster. It’s designed to help you explore scenarios, understand formulas, and document assumptions.
Key features
- Uses the standard normal approximation
- Shows conservative “worst case” p=0.5 sizing
- Highlights when the approximation can break (tiny n or extreme p)
Formula
n ≈ z^2 * p(1-p) / E^2Quick examples
Worst-case sizing uses p=0.5 (max variance).If you halve E, required n roughly quadruples (because of 1/E²).
How to use it (quick steps)
- Pick a confidence level (e.g., 95%) → get z.
- Choose a proportion estimate p (pilot data or conservative p=0.5).
- Choose margin of error E (in probability units, e.g., 0.03 for ±3%).
- Compute n ≈ z²·p(1−p)/E², then round up.
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FAQ
Is this calculator official?
No. GetCalcMaster provides educational estimates and learning tools. Always verify against official definitions, documents, or professional advice.
Do you store my inputs on the server?
No. Calculations run locally in your browser. Optional remember/restore features (if enabled) use local browser storage.
Tip: For reproducible work, save your inputs and reasoning in Notebook.