What proportion should I use if I do not know it?

Use 50%. It gives the largest conservative sample size for a proportion estimate.

What does response rate change?

Response rate does not change the completed sample size. It estimates how many people you may need to invite to get that many completed responses.

Estimate how many completed responses you need for a survey or proportion estimate.

Answer

385

Completed responses needed

Invitations needed: 385

Before population correction: 385

After population correction: 385

Z score: 1.96

Confidence levelMargin of error (%)Expected proportion (%)Population sizeExpected response rate (%)

Confidence

Margin

Scenarios

n0 = z^2 * p(1-p) / e^2; n = n0 / (1 + (n0 - 1) / N)

The calculator first estimates sample size for a large population, then applies finite population correction when population size is entered.

Symbol	Variable	Description
z	Z score	The confidence level multiplier.
p	Expected proportion	Use 50% when the true proportion is unknown.
e	Margin of error	The acceptable error as a decimal.
N	Population size	The total population, if known.

1
Choose confidence
95% is common. Use 99% when you need a stricter estimate.
2
Set margin of error
A smaller margin requires more completed responses.
3
Enter population if known
Finite population correction can reduce the required sample for smaller populations.

These combinations are often used for surveys and polling.

Confidence	Margin	Proportion
95%	5%	50% when unknown
95%	3%	50% when unknown
99%	5%	50% when stricter confidence is needed

Standard Deviation Calculator

Calculate sample or population standard deviation from a raw data list, with variance, mean, standard error, and quartiles.