The wetted perimeter of a channel is a measure of how much of the channel's cross section is touching liquid when the liquid flows through at a certain depth. For example, if water were flowing 1 meter deep through a rectangular channel 3 meters wide, the wetted perimeter would equal the width of the channel's bottom plus 1 meter on each side, or 5 meters. Since all sides of a full pipe are touching water, the wetted perimeter of a circular pipe equals the inner circumference of the pipe.
Things You'll Need
 Wrench
 Measuring tape

Detach one end of the pipe to expose its cross section, if possible.

Measure the inner diameter of the pipe by finding the greatest distance between two points on its circumference. If it is not possible to expose the end of the pipe, measure its outer circumference (Co) and estimate or look up the thickness (T) of the material the pipe is made of. Calculate the inner diameter (D) using this formula: D = (Co / pi)  (2 * T)

Calculate the inner circumference (Ci) of the pipe using this formula: Ci = pi * D, where D equals the inner diameter. The wetted perimeter of the pipe when it is full equals the inner circumference.
References
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