At what pipe size does the nominal sizing change from inside diameter to outside diameter?

The nominal sizing of pipes transitions from using inside diameter (ID) measurements to outside diameter (OD) measurements at 14 inches. This change is applicable primarily to the classification of pipes for various applications, especially in larger sizes where the distinction between the internal and external dimensions becomes significant for purposes such as pressure ratings and fitting compatibility.

In the case of pipes smaller than 14 inches, the nominal size typically refers to the inside diameter, which is often more relevant for flow calculations and fitting alignment. As the pipe sizes increase, the emphasis shifts to the outside diameter, which is crucial for installation and connection with various fittings and supports where wall thickness must also be considered.

Understanding this transition helps professionals select the proper materials and fittings for specific applications, ensuring that all components fit correctly without compromising the integrity or flow efficiency of the piping system.