Request copies of your transcripts from all of the schools you attended. Since you are calculating the GPA for your own purposes, an unofficial or online transcript suffices.
Change the letter grade for every class taken to a number, depending on the grade. An F changes to 0, a D to 1, a C to 2, a B to 3 and an A to 4. Add 0.3 if you got a grade with a plus. For example, a D+ would be worth 1.3. Subtract 0.3 for grades with a minus. For example, a D- would be worth 0.7.
Multiply the number by the credits earned in the class to calculate the points you earned for that class. For example, if you earned a four-credit D+, multiply 4 by 1.3 to get 5.2 points.
Add all of the points earned in each class.
Add the number of credits taken at each school. For example, if you took 56 credits at your first school and 65 at your second, add 56 plus 65 to get 121 total credits.
Divide your total points by your total credits to find your GPA including transfer work. Completing the example, if you earned 260 points in 121 credits, divide 260 by 121 to get 2.15 as your GPA including transfer work.