There are 2 different ways of grading.

1. Relative Grading
2. Absolute Grading

1. Relative Grading (Through Which I studied)

The grades are given on the basis of class average. For example:

The average of the class is 50 and the total marks are 100. So the people near 50 (+ 10 and - 10) depending on instructor would give you a C grade. Above 60 would get C+, Above 70 would get B, Above 80 gets B+ and above 90 gets A. Subsequently below 40 gets a D and below 30 gets a F.

The problem in this is that if the class average is even 60 or 65, there is possibility that you only get a C grade. So it all depends on the class average and the instructor.

2. Absolute Grading:

Its like predefined grades. That you get this much of marks and you get this grade. 90 gets A, 80 gets B+, 70 gets B, 60 gets C+ etc.

How to calculate GPA:

It depends on your credit hours.

Example:

1. The credit hours for Maths are 3.0
and credit hours for English are 1.5.

So what happens is the grades which you get defines the GPA.

D = 1.0
C = 2.0
C+ = 2.5
B = 3.0
B+ = 3.5
A = 4.0

If you get a C+ in Maths means you get 3 x 2.5 = 7
If you get A in English means you get 1.5 x 4 = 6

Now add these, 7 + 6 = 13 and divide by total credit hours, which are Maths + English = 3 + 1.5 = 4.5

13 / 4.5 = 2.89

So GPA = 2.89

Hope it clears your confusions.