Which statement best describes the relationship between nominal pipe size and actual outside diameter in NPT fittings?

• A. Nominal Size Is Smaller Than Actual Outside Diameter
• B. Nominal Size Is The Same As Actual OD For All Sizes
• C. Nominal Size Is Unrelated To OD
• D. Nominal Size Is Larger Than Actual OD

Answer: (A)

In NPT fittings, the nominal pipe size is a label for sizing that reflects the pipe’s approximate bore, not the actual outside measurement of the threaded portion. The external threads are standardized for each nominal size, but their major diameter is larger than the nominal size itself. This means a 1/2-inch nominal NPT, for example, has an actual outside thread diameter of about 0.84 inches. The difference comes from historical sizing conventions and the need to maintain a consistent threading standard across fittings. So the nominal size is smaller than the actual outside diameter.