Glace,
I'd suggest you take a look at ASME/ANSI B16.11 Table 2 "Correlation of Fittings Class with Schedule Number or Wall Designation of Pipe for Calculation of Ratings".
Since you're new to this business, I'll save you some time and let you know that ASME/ANSI codes and standards aren't on the 'net and they're not free. If your company doesn't have what you need, check your local library, or perhaps your pipe supplier has a copy.
Normally the required pipe wall thickness is first determined, and it's designated by either the "weight" (standard weight, extra strong or double extra strong) or more commonly, as GenB mentioned, by the schedule (see ASME B16.10). Originally the schedule number was equal to 1000 times the Barlow formula P/S value, but for various reasons, this is no longer valid. The schedule number is now only a method of indicating wall thickness.
Based on the pipe schedule, the corresponding fitting class can be determined per ASME B16.11 table 2.
Hope this helps and good luck.
Donf