NPT pipe sizes are only loosely related to the actual measurements, so identifying a mystery fitting normally means calipers, an outside-diameter reading and a table. Sean Ragan’s plate holds five pairs of male-to-female adapters in every standard size from 1/8 to 3/4 inch — push each male end through from the front, tighten the female on the back, and from then on you just try the fitting against it.