Body weight

The term body weight is overwhelmingly used in daily English speech as well as in the contexts of biological and medical sciences to describe the mass of an organism's body. Body weight is measured in kilograms throughout the world, although in some countries it is still measured in pounds (e.g. United States) or stones and pounds (e.g. among people in the United Kingdom) and thus may not be well acquainted with measurement in kilograms. Most hospitals, even in the United States, now use kilograms for calculations, but use kilograms and pounds together for other purposes. Body weight of a person is theoretically the weight of the person without any items on. However, for all practical purposes, body weight is taken with clothes on but often without the shoes and heavy accessories like mobile phones and wallets.

In physics, body mass (an expression of matter that does not change due to gravity) is expressed in kilograms while body weight (which is an expression of force that includes gravity) is expressed in newtons.

Stability
The stability of body weight depends on the energy intake and expenditure. When energy intake exceeds output, the excess energy is stored in the body as carbohydrates, proteins or fats and this causes a gain in body weight. The converse is also true. When energy expenditure exceeds energy intake, body weight decreases.

Estimation in children
A number of ways to estimate weight in children have been developed. They include: Broselow tape, Leffler formula, and Theron formula. The Broselow tape is based on length with weight read from the appropriate color area.

The Leffler formula is used for children 0–10 years of age. In those less than a year old it is

$$m = \tfrac{1}{2}a_m + 4$$

and for those 1–10 years old it is

$$m = 2a_y + 10$$

where m is the number of kilograms the child weighs and am and ay respectively are the number of months or years old the child is.

The Theron formula is

$$m = e^{0.175571a_y + 2.197099}$$

where m and ''ay are as above.

Sports usage
Participants in sports such as boxing, mixed martial arts, wrestling, rowing, judo, and weight-lifting are classified according to their body weight, measured in units of mass such as pounds or kilograms. See, e.g., wrestling weight classes, boxing weight classes, judo at the 2004 Summer Olympics, boxing at the 2004 Summer Olympics.