In order to calculate your cumulative average (GPA), you need to know the value of the grades you receive. The grades you receive in your classes are based on the grading scale of each instructor in your classes. Your cumulative average is the resulting value of all of the grades. First assign a grade value to each letter grade. In a 4.5 grading scale, the values go as follows: A+ = 4.5, A = 4.0, B+ = 3.5, B = 3.0, C+ = 2.5, C = 2.0, D = 1.0, and F = 0. Next, you'll need to look at the quality points. This is the number of hours of the course times the grade value. To get your GPA, divide this number by your total number of quality points.