It's not too difficult to calculate if you follow the basic A=4, B=3, C=2, D=1, F=0 rule. For + or -, you add or subtract 1/3.
For example, a B+ counts as 3.33 while an B- is 2.67.
Add those numbers up and divide by the total course hours. You can do the same thing to calculate your science and non-science GPA too. Also, withdrawn courses do not factor into your GPA.