Question: how are npt threads measured?
Answer: As a general “rule of thumb” an NPT thread is approximately 1/4″ (0.25”) larger than its “name.” For a 1/4″ NPT fitting the “nominal” OD is 0.533”. NPT fittings are slightly tapered so the “nominal” diameter is the diameter in the middle of the threaded portion, as measured by the top (crest) of the threads.