You use the Pipe Sizing dialog to automatically specify the sizing for sections of pipe using friction and/or velocity sizing methods.
Revit provides a general conversion from fixture units to flow, using the values found in the 2006 International Plumbing Code (IPC), Table E103.3 (3). The flow conversion method selected in the Properties palette for the selected system determines the section of the IPC table used for the conversion. See Viewing System Properties. The resulting flow is used to calculate pipe sizing.
The value calculated for flow can be helpful in sizing the branch piping for subsystems. However, when specifying sizes for the main piping, you should consider a variety of other factors, including the type of system, the type of building, peak demand, available supply pressure, the pressure required at the highest fixture, and limitations imposed by local authorities, selected fixtures, and supply source. The 2006 International Plumbing Code, Appendix E provides detailed information that must be considered when planning a plumbing system.