Hollow everyday circular objects appear differently than drawn two-dimensional circles. Objects like pipes and hoses have two different diameters. The outside diameter measures the distance of a straight line from one point on the outside of the object, through its center, and to an opposite point on the outside. The internal diameter measures the inside of the object. Calculating the internal diameter depends on the outside diameter and the thickness of the outer circle.
Find the Diameter
Find the total diameter of the object in question by taking a measurement from the outside wall of one side (the point of origination) straight across to the outside wall of the other side (the endpoint). Make sure that your measurement passes through the center of the object and that the point of origination and endpoint are on opposite sides of the object. For the purpose of an example, assume that the object you're measuring is a large pipe that has a total diameter of 40 inches.
Look at Thickness
Determine the thickness of the object. Depending on the object you're measuring, you can do this either by looking up information about the object on a data sheet or by physically measuring the thickness from the outer wall to the inner wall. Keep in mind that because you're measuring only the wall thickness, your measurement shouldn't include any of the space within the object. For the example of our 40-inch pipe, assume that the thickness measures at 2 inches.
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Double the Thickness
Because your initial diameter measurement includes the thickness of the object both at the point of origination and the endpoint of the measurement, it actually passes through the wall of the object twice. To compensate for this, multiply your thickness measurement by 2. For the pipe example, this means you would multiply the 2-inch thickness by 2 to end up with 4 inches of pipe wall as part of your total diameter.
Subtract to Find Internal Diameter
Subtract the doubled thickness from the total diameter to calculate the internal diameter. Doing so removes the object walls from your measurement, leaving only the space between them. In our 40-inch pipe example, the 40-inch diameter includes 4 inches of pipe wall that needs to be removed. Subtracting 4 inches from 40 inches gives us 40 - 4 = 36. This means that the internal diameter of the pipe in our example is 36 inches.
TL;DR (Too Long; Didn't Read)
Use a micrometer if you require an extremely precise measurement.