Sample Size Calculator
Find the sample size needed for a target margin of error and confidence level.
What is Sample Size?
Planning a study runs the margin-of-error formula backwards: n = (z* × σ ÷ E)². It answers how many observations are needed before the data is collected, which is a very different question from analysing data already in hand.
How it works
The large-sample z value is used because the sample size is not yet known, and the exact result is always rounded up — a required n of 166.7 has to be 167 people, and rounding down would quietly miss the target margin. Both the exact and rounded values are shown so the rounding is visible.
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
Realistic sample
Type or paste any realistic input — the tool computes the result locally and instantly.
Common mistakes
Expecting a server round-trip
Nothing is uploaded — every tool in this category runs entirely in your browser.