The term body weight is used in daily English speech as well as in the contexts of biological and medical sciences to describe the Earth's gravitational pull on 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 some people 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.
 Average weight around the world
|Country/Region||Average male weight||Average female weight||Sample population /
|Brazil||72.7 kg (160.3 lb)||62.5 kg (137.8 lb)||20–74||Measured||2008–2009|||
|Chile||77.3 kg (170.4 lb)||67.5 kg (148.8 lb)||15+||Measured||2009–2010|||
|Germany||82.4 kg (181.7 lb)||67.5 kg (148.8 lb)||18+||Measured||2005|||
|South Korea||68.6 kg (151.2 lb)||56.5 kg (124.6 lb)||18+||Measured||2007|||
|UK – Wales||84.0 kg (185.2 lb)||69.0 kg (152.1 lb)||16+||Measured||2009|||
|United States||86.6 kg (190.9 lb)||74.4 kg (164.0 lb)||20–74||Measured||1999–2002|||
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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 for circumstances (such as emergencies) when actual weight cannot be measured. The most commonly used methods include guesses of the child's weight by parents or healthcare providers, weight-estimation formulas based on the child's age and tape-based systems of weight estimation. Some of the many formulas that have been used include the APLS formula, the Leffler formula, and Theron formula. There are several tape-based systems for estimating children's weight, the most well-known of which is the Broselow tape. The Broselow tape is based on length with weight read from the appropriate color area. Newer systems, such as the PAWPER tape, make use of a simple two-step process to estimate weight: the length-based weight estimation is modified according to the child's body habitus to increase the accuracy of the final weight prediction.
The Leffler formula is used for children 0–10 years of age. In those less than a year old it is
and for those 1–10 years old it is
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
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.
 See also
- Bergmann's Rule
- Birth weight
- Body mass index
- Body volume index
- Hesse's Rule
- Human height
- List of the heaviest people
- Weight loss and weight gain
- Weight phobia
- ^ 
- ^ Encuesta Nacional de Salud 2009–2010 (p. 81)
- ^ 
- ^ 
- ^ "The Welsh Health Survey 2009, p. 58". Wales.gov.uk. 2010-09-15. http://wales.gov.uk/docs/statistics/2010/100915healthsurvey09en.pdf. Retrieved 2011-01-22.
- ^ United States National Health and Nutrition Examination Survey, 1999–2002
- ^ a b c d So TY, Farrington E, Absher RK (June 2009). "Evaluation of the accuracy of different methods used to estimate weights in the pediatric population". Pediatrics 123 (6): e1045–51. doi:10.1542/peds.2008-1968. PMID 19482737.
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