Try this scale:
Colleges that you apply to will be very familiar with the IB program and will have no problem interpreting your transcript or converting your IB grades into whatever scale they usually use to evaluate your application.
If you are asking for your own peace of mind, the rough scale goes something like this:
7 --> 4.5
6 --> 4
5 --> 3.5
4 --> 3
3 --> 2
2 --> 1
1 --> 0