Select a graduating class of students. A graduating class is a body a students that are in the same grade level and therefore expected to graduate in the same year. From this class, determine the number of students that graduated with a high school diploma.

Determine the number of students in this graduating class that dropped out. Also determine the number of students that graduated from an alternative certification program. An example of an alternative certification program would be a General Educational Development course, known as GED.

Add the number of students that graduated with a diploma, the number of students that dropped out and the number of students that graduated through alternative means. The sum of the three numbers is known as the "dropout denominator." For example, if you had a graduating class with 900 standard graduates, 75 dropouts and 25 alternate-route graduates, you would add 900 plus 75 plus 25 to get a dropout denominator of 1,000.

Divide the number of students that graduated with a standard high school diploma by the dropout denominator. The result of this division is the graduation rate. Using our example from above, this would give us 900 / 1,000 or a graduation rate of 0.9.

Convert the graduation rate into a graduation percentage. To do this, multiply the graduation rate by 100. Using our example, this would give us a graduation percentage of 0.9 x 100 or 90 percent.

Subtract the graduation percentage from 100. The difference is the dropout rate. Using our example, this would give us 100 - 90, or a dropout rate of 10 percent.