Use a tape measure or caliper
Measure the outside diameter across the widest part of the pipe
Measure the inside diameter if the pipe is open and accessible
Measure the circumference with a tape, then divide by 3.1416 to get diameter
Measure the wall thickness from the edge to the inner surface
Check the pipe’s nominal size marking if available
Compare the measured diameter to standard pipe size charts
Measure the pipe length from end to end if needed
Use the correct unit consistently, such as inches or millimeters