Determining the size of existing black iron pipe is accomplished by measuring the outside diameter of the pipe. Fittings are sized to work with the outside dimension of the pipes involved. It is not recommended that natural gas or other gas fuel plumbing work be performed by anyone other than a licensed professional.
Anyone undertaking such a plumbing project should first consult with local authorities and utilities for specific safety requirements. Determining adequate and appropriate dimensions for a new application requires analysis of gas delivery pressure, the volume of supply required, and the total length of pipe to reach the consumption point. The calculations in this type of analysis are complex and should be conducted by a licensed professional in compliance with all applicable safety regulations.
Generally, the total length of piping to the most remote outlet is used in conjunction with the supply pressure to determine an optimum pipe diameter. Where multiple outlets and appliances are involved, each section and all fittings must also be considered separately, and appropriate sizing may dictate decreasing the diameter of piping from the source to any appliance.
The National Fire Prevention Association NFPA 54: NATIONAL FUEL GAS CODE provides minimum considerations for safe design and construction. Many local regulations, however, are more specific.