Your body mass index is a body fat percentage based on a ratio of your weight to your height.
It is calculated using your body weight divided by your height squared. To calculate your body mass in US units, multiply your weight in pounds by 703. Divide this number by your height in inches squared. The answer should be a number somewhere between 17 and 40. Round to the nearest number.
Body mass index (BMI) is used by physicians to show you how far off your weight is from what is considered “normal,” allowing health care providers to discuss body and weight issues with patients based on a specific number. Your BMI is also used as the basis for individual diagnosis of low weight and obesity.
A person with a “normal” BMI will have a BMI of somewhere between 20 and 25. This simply means that a person is neither underweight nor overweight, so their body weight in comparison to their height is within what is considered to be a healthy range. A breakdown of possible BMI’s and their assigned weight class is as follows:
- less than 16 = severely underweight
- less than 20 = underweight
- a range of 20 – 25 = optimal weight
- greater than 25 = overweight
- greater than 30 = obese
- greater than 40 = morbidly obese
Additionally, a BMI around 15 can show a risk or starvation and that an individual may have other associated health risks. A BMI of lower than 17.5 is informally used as a basis to diagnose anorexia and other eating disorders.
BMI ranges and their indications can differ slightly by age, particularly in children. The same formula is used for children, but the BMI number is compared to other children of the same sex and age in order to determine what is normal, underweight, overweight, and obese, since children tend to weight less in comparison to their height.
A slightly lower BMI for children can still fall within what is considered to be the normal range, while the overweight category will also begin at a slightly lower number. The same applies to the elderly, starting around age 75-80, when people typically begin to weigh less due to eating fewer calories and inactivity associated with age-related concerns.
BMI can also differ around the world according to heritage and culture. For example, in Asian and South Pacific countries, in cultures that tend to experience fewer incidences of overweight issues and obesity, a lower BMI is still considered to be within the normal range, while the numbers to indicate that a person is overweight, obese, or morbidly obese will also be lower than the numbers for North Americans.
BMI statistics have been used to shown an increase in weight and obesity over the last several decades. For example, in 1994, statistics showed that 59% of American men and 49% of American women had a BMI reading over 25, putting them into the overweight, obese, or morbidly obese categories. On average in 1994, 54% of Americans were considered to be overweight. By 2007, 63% of Americans were giving a BMI reading over 25 and considered to be overweight, while 26% were considered to be obese based on their BMI.
Body mass index readings were developed in Belgium between 1830 and 1850 in order to rate a population’s weight, during the development of social physics as a means to study populations. In 1972, during the time at which obesity was becoming a social issue in Western Cultures, The Journal of Chronic Diseases still found body mass index to be “the best proxy for body fat percentage among ratios of weight to height.”
However, body mass index is a controversial measurement because it is widely used as a medical diagnosis for body weight, which was not and is not currently its true intention. BMI readings were developed in order to:
- categorize physically inactive people with an average body composition, and
- for use in population studies, not for giving an individual diagnosis of weight
Body mass index is also controversial and misunderstood because it is defined as a person’s “body fat percentage,” but a person’s BMI does not actually give a measurement of one’s percentage of body fat. Rather, it puts a person into a categorically defined weight class range based on a number that is not a percentage.
For example, a person with a BMI of 23 does not have 23% body fat, but simply falls into a certain weight class based on their height. BMI does not take into account certain factors such as muscle mass, which weighs more than fat and could in fact put a very muscular person into an overweight category for their height when their actual percentage of body fat is very low.
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