# Statistics > Sample Size > 1-Population Proportion

This tool determines the sample size required to estimate a population proportion. You will need to provide the confidence level and desired margin of error. You can also provide a proportion estimate if known. If a proportion estimate is given, the sample size is computed using the following formula:

where zα/2 is the inverse cumulative probability of the z distribution at 1 - &alpha/2, is the proportion estimate, , and E is the margin of error.

If a proportion estimate is not given, the following formula is used:

To use this tool, provide the following inputs to the dialog:

- Confidence Level: a number between 0 and 1. For example, enter 0.95 for a 95% confidence level.
- Proportion Estimate: an estimate of the population proportion.
- Margin of Error: the desired margin of error.