O/D measurement refers to the measurement of the outside diameter of a pipe. Whether you are using a caliper, flexible tape, or some other precision measurement tool, it is important to be exact when taking O/D measurements. The O/D measurement is a part of the nominal pipe size standardized set of sizes for pipes in North America. The specifications have to do with high vs. low pressure and temperatures. Depending on the nominal pipe size, the O/D measurement can be found within standard reference tables using ASME standards. The O/D measurement will always be greater than the nominal pipe size.