The most common scale is:

A+ = 4.0

A = 4.0

A- = 3.666667

B+ = 3.333333

B = 3.0

B- = 2.6666667

C+ = 2.3333333

C = 2.0

C- = 1.6666667

D+ = 1.3333333

D = 1.0

F = 0.0

Thus, if John received two B's and a D one semester, with the B classes being 3 credits each and the D class being worth 4 credits, then John's grade point average for the semester would be (((3.0*3) + (3.0*3) + (1.0*4))/(3+3+4)) = 2.2 GPA.

At most universities and colleges, a GPA of under 2.0 is sufficient
grounds for a student to be placed on academic probation or expelled.

There are numerous variations, such as basing it on a 5.0 scale instead of 4.0 or moving down by increments of 0.2 instead of 0.33333.