A youth's size small has a chest measurement of 30 inches, a waist measurement of 25 inches, and a hip measurement of 31 inches. Those measurement form the standard for a youth size 12 in the fashion industry. These measurements are a guideline, and designers often deviate from them.
The chest measurement is used for tops and jackets, though the hip measurement can be useful depending on the cut of the clothing and child's body type. The waist and hip measurements are used for skirts, shorts and pants.
To get accurate body measurements, wear either a leotard or body suit. Wearing properly fitting undergarments works just as well. Use a flexible cloth tape measure. For the chest measurement, measure straight across the back and around the fullest part of the chest. For the waist, measure around the body at the natural waistline. For the hips, measure around the body at the fullest part of the hips. For all measurements, keep the tape measure level, and wrap it around the body so that it is snug but not too tight.
While not usually a problem for children's clothing, particularly for boys, vanity sizing can be a frustrating problem. In vanity sizing, a size based on standard measurement is given a smaller label size. For example, a standard size 10 might be called a size 8 in one line and a size 6 in another.