calculation for rating

You do not calculate, you select the one that meets or exceeds the requirement with factor of safety.

Find out your application - temperature and pressure. Refer teh ASME for the suitable class for the P,T.

You ma also refer ASME B31 Series, B16.5 for flanges and fittings...

for example piping ASME B31.3 , fittings B16.5, CS Pipes A53B etc

Guest #2

ASME B16.5 is for "Flanges (and FLANGED Fittings) only.

It does Not apply to Butt Weld, Threaded or Socket Weld Fittings as your posting implys.