IQ is calcuated using test results that are compiled and then compared with the results from other people in the same age group to determine the test taker's intelligence quotient. The first step in measuring a person's IQ is to administer a specially designed test.
Although the process has changed slightly, the underlying basis of the procedure used to determine IQ is first administering a test to determine a person's mental age or how much they know compared to others their same age. The mental age is then divided by that same person's chronological age and then multiplied by 100 to determine IQ.
IQ is not a measure of how well someone performs on a test. It is a number that represents the differences in intelligence levels across a large group of people.