you need
• - measuring tape;
• - caliper;
• - Calculator;
• - a sheet of paper and a pencil.
Guide
1
order to measure the outside diameter of the pipe of small size, use a measuring tool, such as a caliper.Slide the tool jaws to dissolve it was larger than the cross section of the pipe.Attach the caliper to the tube and squeeze the sponge tool so that they are tightly covered tube.By measuring scale to determine what the measured diameter of the pipe.Caliper provides accurate measurement of the diameter of the pipe to a tenth of a millimeter.
2
To measure the inner diameter of the pipe, use upper jaw calipers.Insert the sponge into the tube and move them to the sponge tightly against the opposite inner edges of the pipe.By
measuring scale, define the inner diameter of the pipe.Note that the standard caliper measures the pipe diameter up to 150 mm.
3
If you need to measure the diameter of the pipe, without having access to its slice, using construction tape measure or string (depending on the size of the pipe).Measure using a thread or a tape measure the circumference of the pipe (its circumference).Then make a calculation of the external diameter of the pipe by the formula:
D = L / p, where L - length of the pipe circumference, p = 3,14 (number of "pi").
For example, if the circumference of 400 mm outer diameter pipe will be:

D = 400 / 3.14 = 127.4 mm.
4
inner diameter of the pipe calculated by the formula:
D '= D - 2 * t, where D - outer diameter of the tube, and t - wall thickness.
So, for the above example, with a wall thickness of 3 mm tube, the inner diameter of the pipe will be:

D '= 127,4 - 2 * 3 = 121.4 mm.
5
If you have a piece of pipe, and known surface area and length of the segment, the diameter calculate, using the formula of the lateral surface of the cylinder:
D = p * N / S, where N - the length of the tube,S - area of ​​the surface, p = 3,14.

D '= D - 2 * t, where D - diameter of the outer tube, and t - the thickness of its wall.