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📏 Sample Size Calculator

Calculate the minimum sample size needed for estimating a mean or proportion with a given margin of error and confidence level.

Sample Size Calculator

Work out how many observations you need for a survey or experiment given your target margin of error and confidence level, for both means and proportions.

Estimator type

Sample size for estimating a population proportion with a desired margin of error.

Inputs

Confidence level

Desired margin of error (in proportion units)

Estimated proportion (optional)Enter as a decimal between 0 and 1. If left blank or invalid, 0.5 is used for worst case (largest required sample size).

sample size calculatorhow many respondents do I need

Required sample size

Minimum recommended n

385

Rule of thumb

Ensure n × p and n × (1 − p) are both at least 10 for normal approximations.

How to use this tool

Choose whether you are planning to estimate a proportion or a mean.
Pick a confidence level and margin of error that match your study needs.
Enter an estimated proportion or standard deviation to compute the required n.

How many respondents do I need?

Turn your desired precision (margin of error) into a concrete sample size.
See how increasing confidence levels inflates the required n.
Use the output as a planning guide for surveys and experiments.

Assumptions behind the sample size formulas

Uses z critical values for two-sided confidence intervals.
Assumes simple random sampling and approximate normality of the estimator.
Does not apply finite population corrections; for very small populations, you may adjust n manually.

FAQ

Should I express margin of error as a percent or decimal?: Enter the margin of error in the same units as the parameter. For a proportion, 0.03 corresponds to ±3 percentage points.
What if I have no idea about the proportion or standard deviation?: For proportions, using 0.5 is a conservative worst-case choice. For means, you can run a small pilot study to estimate the standard deviation.
Does this handle power or hypothesis tests?: This calculator is focused on confidence interval precision. Power and hypothesis-test sample size planning requires specifying effect sizes and is beyond this simple tool.

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