Change your letter grades into "points." An "A" is worth 4, a "B" is worth 3, a "C" is worth 2, a "D" is worth 1, and an "F" is worth 0. Some schools differentiate between an "E" and an "F." Both are failing grades, but while an "E" is worth "0," an "F" in this case is worth -1. Typically, a "+" adds 0.3, and a "-" subtracts 0.3, so that a B+ is worth 3.3, and so on.
Multiply each converted grade by the number of credits that the course was worth. Most colleges award 1 or 3 credits for a full-time class, and 0.5 or 1.5 for a half-time class, respectively. Courses with labs are often worth more, so check your transcript.
Add up all the products. This is your total number of grade-points.
Add up the total number of credits earned.
Divide this sum by the number of credits you earned. The number should be between 1 and 4. This is the average number of points you earned per credit - your GPA.