From household applications involving natural gas stoves and boilers, to factory applications with industrial machinery, gas pipes are found in many locations. Gas pipes come in numerous sizes and lengths, and if you need to know the diameter of a pipe but can't physically measure the width of one of the pipe's circular ends you can still calculate it manually. By measuring the shortest distance around the pipe -- or circumference -- you can then determine diameter.

Position a levelling tool perpendicular or at a 90 degree angle to the length of the pipe.

Place a tape measure against the level. Wrap the tape measure around the pipe to determine circumference. The level helps you make a straight measurement around the pipe. For our example, assume the circumference is 35.5 inches.

Divide the circumference by the number pi to obtain the pipe diameter in inches. Use 3.1415 for the number pi. Completing the exercise leads to 35.5 inches divided by 3.1415 or a diameter of 11.3 inches.