How Can I Calculate my GPA Myself for College?

I want to know how can I calculate my college GPA. I've tried to calculate it with GPA calculators, but they all have a + or - grading scale, which is not what my college does, so I'm having trouble to do it myself and I can't find any help. The one I need should be done according to the number of credit hours correspondent to each class. Does anyone know a method to calculate my GPA myself for college taking into account the specifications I've just made?

Here's how to calculate your GPA for college. You have to bare this in mind to calculate your college GPA: A = 4, B = 3, C = 2, D = 1, F = 0. For example, if a class is 3 credits and you get an A, then it's 3x4 = 12 grade points. For 15 hours (which means 5 - 3 credit classes): you get 2 - A's, 2 - B's and 1- C then you have to calculate this: 4x3 + 4x3 + 3x3 + 3x3 + 2x3 which is 12 + 12 + 9 + 9 + 6 = 48 grade points. Finally, you must divide it by the 15 credit hours, this means that your college GPA is 3.2.