Determine the GPA to percentile equivalence criteria established by your institution.
Open the GPA Conversion Chart given at McMaster University's website. This conversion chart lists the different equivalence criteria for converting from GPA to percentage for different institutions in separate columns. Look up your institution's equivalence criteria under the "Percentage" tab of the chart.
Figure out the range of percentile values in which your GPA lies. Assuming that your institution's equivalence criteria is listed in column 4 of the table, look up your GPA score in the leftmost column and see the corresponding percentile range in column 4 of the table. If your university's equivalence criteria were to fall in column 3 of the table, then you would look up the GPA score given in the leftmost column against the corresponding percentile range given in column 3 of the table.