Use a tape measure or caliper
Measure the outside diameter of the pipe
Measure the inside diameter if the pipe end is open
Measure the circumference and divide by pi if needed
Check the pipe’s nominal size marking if available
Compare the measurement to a pipe size chart
Identify the pipe material before matching sizes
Measure in inches or millimeters as required
Measure across the widest point of the pipe
Record the wall thickness if exact sizing is needed