Calliper: A calliper is the most accurate way to measure diameters.
2. Pipe wrench: Place a pipe wrench around the tube tightly and place a ruler or tape measure along the jaw opening to read the distance and thus measure the diameter of the tube.
3. String: Tie a string, thread or ribbon tightly around the tube and cut it. Next, measure the length of the string precisely. This will determine the circumference of the tube. Divide the measured circumference by 3.14 to calculate the tube diameter.