Identify the weight given to each letter grade in your school. For regular classes in most schools, a grade of "A" is worth 4 points. In AP classes, an "A" is worth 5 points. But due to large differences between school districts, you must ask a teacher to find out the correct weight per letter grade.
Add up the points each letter grade is worth, according to the correct weight.
Divide the total grade points from all your classes by the number of classes you took. For example if you took four classes and you got 17 total grade points, then you have a GPA of 4.25 (17/4 = 4.25).