Galvanised metal is material that has been subjected to an electrochemical procedure known as galvanic current, which provides the metal with high endurance properties. Just like with pipes of other metals, galvanised pipes are sized according to a standard measuring table based on the outside diameter of the pipe itself. There are other sizes though that are not standardised, like the inside diameter and wall thickness, that you might occasionally need to know.
Clean the pipe thoroughly on the outside and the inside to remove any dirt or build-up that can interfere with your measurements. Even 1/8 inch of dirt can render your measurement inaccurate.
Wrap the measuring tape around the outside of the pipe and pull the tape taught. Check the number where the end of the tape meets the rest to find the circumference of the pipe.
Divide the circumference of the pipe by 3.14 to find the outside diameter of the pipe. Find the diameter on the nominal pipe size chart and check the corresponding number to find the standardised size of your pipe.
Hold the measuring tape around the inside of the pipe to find the inside circumference and then divide the number by 3.14. The quotient is the inside diameter of the pipe.
Place the measuring tape so that the end is at the outermost edge of the pipe. The length of the tape should be trailing down across the middle of the pipe opening.
Check the measuring tape where the wall of the pipe ends and the inside of the pipe begins to find the wall thickness. Use the thickness to find the pipe gauge on the pipe gauge chart.
Measure your pipes when they are new if possible to ensure the most accurate readings.
Do not measure broken or deformed areas of pipe to be used as a reference because the reading won't be good for the rest of the pipe.