Sample Size Calculator

The confidence level of the planned interval, as a percentage such as 95. Higher confidence requires a larger sample.

The largest acceptable distance between your estimate and the true value — the ± half-width of the interval.

For a mean, use the same units as the data; for a proportion, a fraction such as 0.05 for ±5 percentage points. Halving the margin roughly quadruples the required sample.

Your best guess of the population standard deviation, e.g. from a pilot study or earlier research. More variability means a larger required sample.

Your best guess of the true proportion. It only affects the variability p(1 − p), which peaks at 0.5 — so leaving 0.5 when unsure never underestimates the required sample.