Margin of Error Calculator
Find the margin of error for a survey or poll from the sample size and observed proportion, at 90, 95, or 99 percent confidence.
Margin of error (± percentage points)
3.10
Breakdown
- CI lower bound (%)
- 46.90
- CI upper bound (%)
- 53.10
- Finite population correction factor
- 1
Last updated:
FAQ
Why is 0.5 the worst case?
The product p(1−p) peaks at exactly 0.5, giving the widest interval. Pollsters use it when planning a sample size before any data exists, because it guarantees the margin will not be underestimated.
Why do polls of ~1,000 people have ±3% error?
At 95% confidence with p = 0.5 and n = 1,000, the margin is 1.96 × √(0.25/1000) ≈ 3.1%. That is the sweet spot national polls have used for decades — doubling precision would require four times the interviews.
What is the finite population correction?
When your sample is a noticeable fraction of a small population — say 300 of 1,000 customers — sampling without replacement is more precise than the infinite-population formula assumes. The correction factor shrinks the margin accordingly.
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