What is the schedule of 16 gauge pipe?

16 gauge pipe refers to the wall thickness of the pipe exclusive of its diameter.
It is 0.064 inches thick, this is a steel wire gauge measurement.

Pipes are typically sold by schedule numbers. A wall thickness of 0.064 inches would correspond to pipe schedule 40 for 1/8" diameter pipe (0.068"), schedule 10 for 1/4" and 3/8" pipe (0.065") and schedule 5 for pipe diameters 1/2" to 2" (0.065"). Over 2" diameter pipe, schedule 5 (the smallest wall thickness) is thicker than 16 gauge steel.