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The diameter of a pipe is the distance between one wall of the pipe to the other wall of the pipe going through the center of the pipe. If you cannot access the end of the pipe, you must measure the circumference of the pipe with a string and then determine the diameter from that measurement. A string allows you to get an accurate reading of the outside of a pipe so you can measure the string with a measuring tape.
Place the tip of a string onto the pipe and hold it in place with one hand. Wrap the string around the pipe until the string reaches the tip on the end of the string you are holding in place.
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Mark the point where the string touches the tip with a marker.
Lay the string onto a ruler with the tip located at the "0" end of the ruler. Stretch the string out across the length of the ruler and then locate the mark on the string. Read the measurement on the ruler. This tells you the circumference of the pipe. For this example, let us say the measurement is 6 inches.
Divide your circumference measurement by pi, which is 3.14159. Given the example of a 6-inch circumference, the diameter of the pipe is 1.909 inches.