How To Calculate Glycemic Index From Food Labels | dLife You can find the standard or predetermined GI of such component on a GI table or database. For instance, the GI of oatmeal on such table is 83. Still using our meal above, multiply 0.46 by 83 to get 38.18 for oatmeal and 0.54 by 46 to get 24.84 for orange juice. Step 4: Add the result you obtained in step 3 together.