First you find your combined average across the board. Lets say your average is 500. You need to calculate a percentage out of the total points possible (usually 800). 500/800= 0.625. Next you need to figure out which grading scale you wish to use (generally a 4.0). You would multiply your percentage by 4 to get your gpa. 0.625*4=2.5. Therefor your gpa would be 2.5. However it is not common for universities to use this method as the only assessment of your academic quality. Generally they will also look at the average total test results for that year and compare your results to the results of others, using a 1 to 99 percentile rating. It is wise to contact the university admissions office where your are applying to determine how they will calculate your academic standing, as it will vary.