You will need
  • measuring ruler or measuring tape;
  • - caliper;
  • - camera.
If you want to determine the pipe diameter, the cross section of which is available to direct observation and measurement, and accuracy requirements measurement minimum use a metal measuring tape or a ruler. Place the measuring tool to the end of the tube at its widest part and count the number of divisions corresponding to the diameter. This method allows you to set the size of the product with an accuracy to several millimetres.
To measure the outer diameter of the small pipe size, use the caliper. Attach the legs to push the instrument to the end of the tube and slide them so that they tightly clung to the walls. On the scale of values, determine the required diameter with a precision of tenths of a millimeter.
If the mechanical part of the pipe is inaccessible for measurements, for example, when a pipe is included in the current system, under operation, a caliper for measurement attach to the lateral surface of the product. This way you can measure the diameter of the pipe, if the length of the legs of the device exceeds half of the diameter.
To calculate the diameter of the larger pipe, use the formula known from the geometry course:

D = L / P; where
D – diameter;
L is the length of the circle;
P is the number PI, approximately equal to 3.14.

To start by means of a cord or tape measure, make the measurement of the pipe on its circumference to find the circumference. The resulting value of the divide by 3.14; the result you will get the diameter of the pipe.
If you hold a direct measurement of the pipe for some reason is not possible, use the copy method. To do so, attach to the pipe measuring tool (a ruler) or piece, the linear dimensions which is known in advance (matchbox). Then take a picture of a pipe section with a measuring instrument. Further measurements and calculations spend on photography. It is necessary to measure in the picture the visible thickness in millimetres and convert the calculated data to the actual size of the pipe, given the scale of the shooting.