Sample Size Calculator
Calculate the minimum sample size for a survey or A/B test given desired confidence level and margin of error.
Inputs
Sample size needed
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For unlimited population
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Z-score used
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Target margin of error
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Confidence level
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About Sample Size Calculator
Common sample sizes
±5% at 95% confidence: 384 (rounded to 400). ±3% at 95%: ~1,067. ±1% at 95%: ~9,600. National polls of 1,000 routinely report ±3% MOE.
Why p = 0.5 is worst case
Variance is maximized at 50/50 split — you need the most samples when you have no prior. If you have a prior (e.g., last poll was 60/40), use that to reduce the required sample size.
A/B test sample size
Different formula — depends on the minimum detectable effect (MDE), baseline conversion rate, and power. Use a dedicated A/B test calculator for accurate numbers.