Sample Size for a Mean Calculator
For measuring an average rather than a percentage.
Sample size needed
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Using z only —
Reading this —
The formula
n = (t × s ÷ e)²
t depends on n, so n is raised until the margin is met. Using z alone gives the simpler textbook figure, which runs low for small samples.
What to watch out for
If you do not know the standard deviation, a small pilot study is the usual way to get an estimate.
Frequently Asked Questions
What is a worked example?
With a standard deviation of 15, estimating to within ±2 at 95% confidence takes 219 measurements.
Why is the answer above the z formula?
The t value is larger than z when the standard deviation is estimated, which matters most in small samples.