Free Sample Size Calculator Online
This free online sample size calculator determines the number of respondents you need for statistically significant survey results. Set your confidence level and margin of error.
Survey Parameters
Use 50% if unsure — gives largest sample.
Required Sample Size
Enter parameters above
How the Sample Size Calculator Works
The sample size calculator uses the standard formula: n = (Z² × p × (1−p)) / E², where Z is the z-score for your confidence level, p is the estimated proportion (default 0.5 for maximum variability), and E is the desired margin of error. For finite populations, the sample size calculator applies a correction factor.
At 95% confidence, the z-score is 1.96. With a 5% margin of error and 50% proportion, the sample size calculator gives n = 385 (for a large population). If your population is 1,000, the corrected sample size drops to about 278.
The sample size calculator supports confidence levels of 90% (z=1.645), 95% (z=1.96), and 99% (z=2.576). Higher confidence means a larger sample. Tighter margins also increase the sample size needed.
When You Need a Sample Size Calculator
Surveys and polls need a specific number of responses to be reliable. Sending a survey to too few people gives noisy results; sending it to everyone wastes resources. The sample size calculator finds the sweet spot.
Clinical trials and A/B tests also require sample size planning. A drug trial needs enough participants to detect a treatment effect. An A/B test needs enough visitors to distinguish a real conversion difference from random noise. The sample size calculator helps with both scenarios.
Quality control in manufacturing uses sample size calculations to determine how many items to inspect from a production batch. The sample size calculator adapts to any population size and desired precision level.
Tips for Using the Sample Size Calculator
Default to 50% Proportion
If you do not know the expected proportion, use 50% — this gives the largest (most conservative) sample size from the sample size calculator.
95% Confidence Is Standard
Most surveys use 95% confidence and 5% margin of error. Start there unless you have a specific reason for higher precision.
Small Populations Matter
For populations under 20,000, enter the actual population size. The sample size calculator will reduce the required sample using the finite population correction.
Add a Buffer
Not everyone will respond. If you expect a 60% response rate, divide the sample size calculator result by 0.6 to get the number of invitations to send.
Frequently Asked Questions
More Math Calculators
Percentage Calculator
Calculate percentages, percent change, and percent of a number.
Percent Change Calculator
Calculate the percentage change between two values — increase or decrease.
Percentage Increase Calculator
Calculate the percentage increase from one value to another.
Fraction Calculator
Add, subtract, multiply, and divide fractions instantly.