Most colleges assign students grades based on their performance in each class. Each semester, these grades are converted to a numerical form, also known as your grade-point average, to calculate how well you did in all of your classes put together. You may have a scholarship that requires that you keep a certain GPA or may be trying to avoid getting below a certain GPA each semester so you're not on academic probation. To calculate your semester average, you need to know your grades and how many credits each class was worth.

Check your school's system for calculating your semester average to determine how many points each letter grade converts to. Most schools give four points for an "A," three points for a "B," two points for a "C," one point for a "D" and zero points for an "F." Some schools add 0.33 points for a "+" and subtract 0.33 for a "-" so an "A-" would be 3.67.

Convert each of your letter grades to a numerical value based on your school's GPA system. For example, using the standard GPA system, if you had an "A-," a "B+," a "C" and a "C-," you would convert those to 3.67, 3.33, 2 and 1.67.

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Multiply the numerical value of each grade by the number of credits each class was worth. Continuing the example, if your first two classes were four credits each and your last two were three credits each, you would multiply 3.67 by 4, 3.33 by 4, 2 by 3 and 1.67 by 3 to get 14.68, 13.32, 6 and 5.01.

Add the values from Step 3 to calculate your total points earned for the semester. In this example, you would add 14.68, 13.32, 6 and 5.01 to get 39.01.

Divide the result from Step 4 by the number of credits you took for the semester to calculate your semester average. For this example, you would divide 39.01 by 14 (two four-credit classes and two three-credit classes) to find your semester average would be about 2.79.