What is the Sample Size Calculator?

Calculate required sample size for surveys, mean estimates, A/B tests, and two-group comparisons with this free sample size calculator. Set confidence level, margin of error, power, and effect size — get step-by-step formulas and rounded-up n. Runs instantly in your browser.

How to use the Sample Size Calculator

Choose Proportion (survey), Mean, Two Means, or Two Proportions (A/B test).
Enter estimated proportion or standard deviation, margin of error, or effect size.
Set confidence level (90/95/99%) or power and significance for hypothesis tests.
Review the recommended sample size per group or total respondents.
Copy calculation steps for your study plan or statistics homework.

Common use cases

Planning survey respondent counts with margin of error
Sizing clinical or psychology experiments comparing two means
Calculating A/B test traffic requirements for product teams
Estimating observations needed for a mean confidence interval
Teaching power analysis in introductory statistics courses

Frequently asked questions

How do I calculate survey sample size?: Use n = (Z² × p × (1−p)) / E² where p is the estimated proportion, E is the margin of error (as a decimal), and Z is the Z-score for your confidence level (1.96 for 95%).
What sample size do I need for an A/B test?: Enter your baseline conversion rate and the minimum absolute lift you want to detect. Set power (typically 80%) and significance (typically 5%). The calculator returns n per group.
What if I don't know the population proportion?: Use 50% for a conservative (maximum) sample size estimate. This assumes the highest possible variability p(1−p).
What is statistical power?: Power is the probability of detecting a true effect. 80% power is standard — meaning an 80% chance of finding a significant result if the effect exists.
What is finite population correction?: When sampling without replacement from a small known population N, the required sample size can be reduced using n_adj = n / (1 + (n−1)/N). Enter population size in proportion mode to apply this.

More Mathematical Tools

How to use the Sample Size Calculator

Choose Proportion (survey), Mean, Two Means, or Two Proportions (A/B test).

Enter estimated proportion or standard deviation, margin of error, or effect size.

Set confidence level (90/95/99%) or power and significance for hypothesis tests.

Review the recommended sample size per group or total respondents.

Copy calculation steps for your study plan or statistics homework.

Frequently asked questions

How do I calculate survey sample size?

Use n = (Z² × p × (1−p)) / E² where p is the estimated proportion, E is the margin of error (as a decimal), and Z is the Z-score for your confidence level (1.96 for 95%).

What sample size do I need for an A/B test?

Enter your baseline conversion rate and the minimum absolute lift you want to detect. Set power (typically 80%) and significance (typically 5%). The calculator returns n per group.

What if I don't know the population proportion?

Use 50% for a conservative (maximum) sample size estimate. This assumes the highest possible variability p(1−p).

What is statistical power?

Power is the probability of detecting a true effect. 80% power is standard — meaning an 80% chance of finding a significant result if the effect exists.

What is finite population correction?

When sampling without replacement from a small known population N, the required sample size can be reduced using n_adj = n / (1 + (n−1)/N). Enter population size in proportion mode to apply this.