IF YOU DO NOT WORRY ABOUT VELOCITY, IT DEPENDS UPON WHERE THE LINE IS COMNNECTED . FOR EXAMPLE (HYPOTHETICAL) IF YOU ARE CONNECTING THE PIPE BETWEEN TWO TANKS WITH DIFFERENT HEAD THE FLOW THROUGH PIPE DEPENDS UPON DELTA OF HEAD AND DISTANCE BETWEEN TNAKS. GO TO ANY HYDRAULIC BOOK TO FIND THE...
IF YOU ARE IN DESIGN START WITH CALCULATION FOR WALL THICKNESS. YOU CAN ALSO CHECK "PROCESS INDUSTRY PRACTICE"(PIP) WHICH HAS SOME EXPLANATION ON ASME B31.3