I don't know of a website with a calculator, but this is copied from our undergraduate catalog and tells how to calculate a GPA manually:
The Grade Point Average (GPA). The grade point average is
a numerical value which is obtained by dividing the number of
grade points earned by the number of hours attempted. This
average, often called the index, is computed at the end of each
semester or term, and on a cumulative basis. The grade of A
represents four points for each hour of credit; B+ three and onehalf points; B three points; C+ two and one-half points; C two points; D+ one and one-half points; and D one point. No points are recorded for an F or a WF, although the hours attempted are included in the computation.
Suppose that the student has earned the following grades:
3 hours of A (equal 12 points)
3 hours of B (equal 9 points)
3 hours of C (equal 6 points)
2 hours of D (equal 2 points)
2 hours of F (equal 0 points)
The semester average, then, would be 2.23, which is the quotient
obtained by dividing 29 (the number of points) by 13 (the
number of hours attempted).
