What is the outside diameter (O.D.) of a nominal 3" pipe?

The nominal size of a pipe does not directly correspond to its actual outside diameter. For a nominal 3-inch pipe, the standard specifications established by organizations such as the American National Standards Institute (ANSI) indicate that the actual outside diameter is typically 3.5 inches. This is due to the pipe's wall thickness and the standardization process that sizes pipes. Nominal sizing is a general reference and helps in identifying pipe sizes, but the real dimensions often differ.

In the options provided, the choice of 3 1/2 inches aligns with the actual measurements for a nominal 3-inch pipe, confirming the correct understanding of how pipe sizing works in practical applications.