5% is a common choice

%

The margin of error is the amount of error that you can tolerate. If 90% of respondents answer yes, while 10% answer no, you may be able to tolerate a larger amount of error than if the
respondents are split 5050 or 4555.
Lower margin of error requires a larger sample
size.

Typical choices are 90%, 95%, or 99%

%

The confidence level is the amount of uncertainty you can
tolerate. Suppose that you have 20 yesno questions in your survey. With a
confidence level of 95%, you would expect that for one of the questions (1
in 20), the percentage of people who answer yes would be more than
the margin of error away from the true answer.
The true answer is the percentage you would get if you exhaustively interviewed everyone.
Higher confidence level requires a larger sample size.

If you don't know, use 20000


How many people are there to choose your
random sample from? The sample size doesn't change much for populations larger than 20,000.

Leave this as 50%

%

For each question, what do you expect the results will be? If
the sample is skewed highly one way or the other,the population probably
is, too. If you don't know, use 50%, which gives the largest
sample size. See below under More information if this is
confusing.

Your recommended sample size is 

This is the minimum recommended
size of your survey. If you create a sample of this many people and
get responses from everyone, you're more likely to get a correct
answer than you would from a large sample where only a small percentage of
the sample responds to your survey. 