Sample size calculator
Find how many respondents you need for statistically significant results. Set confidence level, margin of error and population size.
Research parameters
Total people in target audience. Leave empty if unknown or very large.
Probability that the true result falls within the confidence interval.
Maximum deviation of results from true values. Standard is 5%.
If unknown, use 50% — gives maximum (conservative) sample size.
Assumptions and limitations
- Cochran formula for proportion estimation.
- p = 50% gives maximum sample size (conservative).
- FPC correction applied when population is known.
- Assumes simple random sampling.
What it means
With 95% confidence level and ±5% margin of error you need to survey at least 385 people for statistically significant results.
In SurveyNinja, NPS, CSAT and more are calculated automatically. Create a survey in 5 minutes and get real-time analytics.
Start freeData is not stored — calculation runs in your browser.
How sample size is calculated
Cochran formula
n₀ = (z² × p × (1−p)) / e², where z is the z-value for confidence level (1.645/1.96/2.576), p is the proportion, e is the margin of error in decimal form.
Confidence level
The probability that results reflect reality. At 95%, if you ran 100 surveys, in 95 cases the result would fall within the confidence interval.
Margin of error
Defines result precision. With MOE ±5% and a result of 60%, the true value lies in the interval 55–65% at the given confidence.
FPC correction
n = n₀ / (1 + (n₀−1)/N). Applied when population N is known. Reduces required sample, especially when n₀ > 5% of N.
Sample size table
* At 95% confidence and p = 50%
What to avoid
- Confusing sample size and population Sample is who you survey; population is the entire target audience. The calculator shows how many people from the population to survey.
- Stopping data collection early Until you reach the calculated size, margin of error will be higher than stated. Reach the target sample size.
- Ignoring FPC for small populations If the audience is a few thousand or less, enter it in the Population field — otherwise the required sample will be overstated.
- Assuming 385 is always enough Size depends on margin and expected proportion. For ±3% or 99% confidence you need more respondents.
Before launching the survey
- Define the target audience and how you will reach them.
- Calculate sample size (this calculator) and fix margin and confidence level.
- Estimate response rate and calculate how many invites to send.
- Prepare a short survey and test invite.
- Launch collection and monitor progress toward the target sample.
FAQ about sample size
What is sample size?
How many respondents do I need for a survey?
Why does p = 50% give the maximum sample size?
What is confidence level?
What is margin of error?
How does population size affect sample size?
What formula does the calculator use?
Can I collect the sample in SurveyNinja?
Automate your metrics
In SurveyNinja, NPS, CSAT and more are calculated automatically. Create a survey in 5 minutes and get real-time analytics.
Start free