Assign number values to all letter grades you have received. Write down the letter grades you have received in every class and beside each letter, assign a number value. Typically, an A equals 4 points, a B is 3 points, a C is 2 points, a D is 1 point and no credit is assigned to an F.

Add all the numbers up to equal one large number. Many people like to add the total for each semester separately, then calculate the overall GPA once they have found the GPAs for each semester. This is easier because you will have smaller numbers to work with, but it is important to keep very accurate records on paper if you choose this method so that you do not wind up mixing scores from different semesters prematurely, thus skewing your final number.

Divide this large number by the amount of classes taken. For example, if you took five classes in one semester and received one A, three Bs and one C, the calculation would be 4+3+3+3+2. This equals 15, which would then be divided by the total number of classes (five) to get 3. Your GPA would then be 3.0.

Add all GPAs for concurrent semesters and divide them by the total number of semesters to calculate your overall GPA. For example, if you had eight semesters in high school and received GPAs each semester of 3.2, 4.0, 3.8, 3.1, 2.0, 4.0, 3.6 and 3.8, you would need to add all these GPAs up and divide them by the total number of semesters (eight) to get an overall GPA of 3.44. It is important to note that GPAs may often have three digits, but if you have done your calculations correctly, the first number should always be a single digit.