For the average 18-year-old female, the bicacromial or shoulder width is about 14 inches or 35 cm. For comparison, the average biacromial width is about 15 inches or 39 cm. for males of the same age. Longitudinal studies show that women experience a growth spurt in shoulder width in their early teens, but that shoulder width does not significantly change after age 18.
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Shoulder width is one of the many measurements used to characterize human body structure. Wide, or broad, shoulders are often considered to be an indication of athleticism, particularly when matched with a narrow waist. Psychological studies indicate that shoulder width, along with waist circumference, is one of the determining characteristics for female attractiveness.
The Biacromial Diameter
Almost every structural aspect of the human body has been studied and quantified, including shoulder width and the branch of anatomy that attempts to quantify structural qualities of the human form is anthropometry. The anthropometric measure of shoulder width is taken from the bony projections, or acromial processes, on the outer tips of your shoulder blades and is called the biacromial diameter or biacromial width.
Another measure used by anthropometrists that involves shoulder width is Tanner's Index of Androgyny. The index of androgyny is calculated from measurements of shoulder width and hip width. A body type demonstrating wide shoulders and narrow hips would have a high androgyny index. A typical nonathletic woman would have an average androgyny index of around 80.
The Real Deal
Although most measurements of shoulder width are reported using the biacromial width based on skeletal structure, shoulder width can vary as well because of the size of the deltoids muscles, which sit on the sides of the shoulder and because of varying thickness of the layer of subcutaneous fat found under the skin. Because of the added width contributed by these soft tissues, the average adult female shoulder width is just under 17 inches or 43 cm.