Sample size calculator
How many responses do you need?
Enter your confidence level, the margin of error you'll accept and, if you know it, your population. We tell you how many responses you need for the results to be reliable within that margin.
Parameters
Leave empty if unknown or very large.
How sample size is calculated
The calculation uses Cochran's formula: the size depends on your confidence level (how likely you want the true result to fall within the margin), your margin of error (how much deviation you'll accept) and the expected variability, which we set at 50% to require the most responses and never fall short.
If you give the population size, we apply the finite-population correction: for a small group you need proportionally fewer responses. Without a population we assume it's very large and the number doesn't drop for that reason.
One important caveat: the size only delivers that margin if you select respondents without bias. A large but skewed sample does not represent your population however many responses it gathers; representativeness comes from random sampling, not from the count alone.
Frequently asked questions
- What confidence level and margin of error should I use?
- The industry standard is 95% confidence with a ±5% margin. Raise confidence to 99% or drop the margin to 3% for more precision, at the cost of needing more responses.
- Do I need to know the population size?
- No. If you don't know it, leave it empty and we calculate for a very large population, the most demanding case. Knowing it can only reduce the number you need.
- What if I can't reach that many responses?
- You can still run the survey: the results will be indicative, with a wider margin of error. Use the margin-of-error calculator to see how much precision you have with the responses you do get.
Where the formula comes from
- Cochran, W. G. (1977). Sampling Techniques, 3rd ed. WileyThe sample size formula and the finite population correction this calculator uses.
- Krejcie, R. V. & Morgan, D. W. (1970). Determining Sample Size for Research ActivitiesThe classic sample size table by population, which yields the same figures.
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