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 outside diameter of the object in question. For this example, the outside diameter is 40 inches.
Determine the thickness of the object, and multiply it by 2. In this example, the thickness of the object is 2 inches. Two multiplied by 2 equals 4.
Subtract the doubled thickness from the outside diameter to calculate the internal diameter. For this example, 40 less 4 equals 36. The internal diameter in this example is 36 inches.
TL;DR (Too Long; Didn't Read)
Use a micrometer if you require an extremely precise measurement.