Margin of Error Calculator
Estimate an uncorrected normal-approximation margin for a sample proportion and confidence level.
What would you like to do next?
Connect sample size with proportion precision
Choose a confidence level, enter a completed sample size and provide the observed proportion. The calculator estimates standard error, multiplies it by the selected normal critical value and displays a margin plus bounded lower and upper percentages. Keeping the standard error visible helps users see why a larger sample usually narrows an otherwise comparable interval.
Check whether the approximation matches the sample
This quick calculation does not apply a finite-population correction, continuity correction, survey weights or a complex-sample design effect. Very small success or failure counts can make the simple normal interval unreliable, and nonrandom recruitment can create bias that a narrow margin never repairs. Match the interval method to the study design and distinguish sampling uncertainty from other sources of error.
Frequently asked questions
What inputs determine the margin?
The selected confidence level, sample size and observed proportion determine this estimate.
Is population size included?
No. This page reports an uncorrected large-population approximation.
Does a small margin prove the sample is representative?
No. Selection and measurement bias can remain even with a narrow sampling margin.