In most of the universities, GPA is calculated on a scale of 4 but they also can add 1 more point for rigorous classes. If you score A in normal class, then it would be calculated as 4 and if you scored A in IB then it would be calculated as 5. A general rule followed by some of the universities and colleges is as below

Translating IB score of 1-0 into 100 pointer scales is
7->100
6->92
5->84
4->76
3->68
2->60
1->terrible

Above score is further translated as follows
92->4.0
84->3.0
76->2.0
68->1.0
60->0.0

Depending on the difficulty of the subject, you can add one as most of the universities do.

GPA stands for Grade Point Average and there is nothing to do with conversion of IB score into GPA. IB score is calculated on a 7 pointer scale whereas GPA is calculated on a 4 pointer scale. Most of the universities simply ask for your IB score and do the GPA conversion on their own.

There is no set rule for converting IB score into GPA. Most of the universities convert IB score as per their score method. So, relax and forget the task of converting IB score into GPA, leave it to your desired university and concentrate on your IB program.