For piping cross sections over 12 inches, sizing is based on what?

For piping cross sections over 12 inches, sizing is based on the outside diameter. This standardization allows for consistency across various applications, ensuring that the piping fits correctly within systems and can be readily compatible with fittings, flanges, and other components that are designed to accommodate specific outside diameters.

Using the outside diameter for sizing is particularly important in industrial settings where large pipes must connect seamlessly with equipment and other piping. The outside diameter helps define how the piping will integrate with the system's connection points, which are typically designed around that measurement.

In contrast, other methodologies such as wall thickness or inner diameter do not provide the necessary context for fitting compatibility and installation alignment, which are critical when dealing with larger piping systems where clearances and support structures can vary significantly. Similarly, while the weight of the material might inform decisions about material selection or handling, it does not directly influence the sizing of pipes in a mechanical sense.