Gather all of your report cards for the period of time you want to average. This will probably amount to a lot of documents since you most likely received a letter grade for each class at the end of each semester, whether you're trying to average your high school or college GPA. You will need the number of credits, or hours, each course was worth and the letter grade you received.
Give each letter grade a number value. The most common way to assign this number value is the four-point system where A = 4, B = 3, C = 2 and D = 1. Some GPA scales also assign points to plus (+) and minus (--) grades. Find out if the college or requesting party wants you to assign different values for plus and minus grades. If so, you can use this scale: A = 4, A-- = 3.7, B+ = 3.3, B = 3, B-- = 2.7, C+ = 2.3, C = 2, C-- = 1.7, D+ = 1.3, D = 1 and D-- = .7. If the school or potential employer wants you to assign points for plus grades only, then assign .5 for every plus grade you receive--for example, a B+ would be worth 3.5. You receive no points for an F or an incomplete.
Add all of the credits completed (including the credits for courses failed). Add all of the grade points earned.
Divide the total of your grade points by the total credits you attempted. This is your overall GPA. The highest possible GPA is a 4.0.