I/D measurement refers to the measurement of the inside diameter of a pipe. You can measure the inside diameter of a pipe using tape, a caliper, and any other type of precision measurement tool. Depending on the size of a particular pipe, there is a standardized corresponding I/D measurement. As long as you know the I/D measurement, O/D measurement, or circumference of a pipe, you should be able to calculate or look up the corresponding values. The amount of ovality that is expectable for a standard pipe is determined by the I/D measurement, and usually ranges between 5 and 10%.