Body Mass Index
- BMI Calculator
- What iS BMI?
- BMI Categories
- The BMI Controversy
To calculate the body mass index (BMI) use our BMI Calculator. The BMI Calculator uses the following formula to calculate the BMI:
What is BMI?
The body mass index (BMI), or Quetelet index, is a statistical measurement which compares a person’s weight and height. Though it does not actually measure the percentage of body fat, it is a useful tool to estimate a healthy body weight based on how tall a person is. Due to its ease of measurement and calculation, it is the most widely used diagnostic tool to identify obesity problems within a population. However, it is not considered appropriate to use as a final indication for diagnosing individuals. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing “social physics”. Body mass index is defined as the individual’s body weight divided by the square of his height. The formulas universally used in medicine produce a unit of measure of kg/m2. BMI can also be determined using a BMI chart, which displays BMI as a function of weight (horizontal axis) and height (vertical axis) using contour lines for different values of BMI or colors for different BMI categories:
Click on the graph below to enlarge it
A graph of body mass index is shown above. The dashed lines represent subdivisions within a major class. For instance the “Underweight” classification is further divided into “severe,” “moderate,” and “mild” subclasses, based on World Health Organization data.
A frequent use of the BMI is to assess how much an individual’s body weight departs from what is normal or desirable for a person of his or her height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight). The World Health Organization (WHO) regard a BMI of less than 18.5 as being underweight indicating possible malnutrition, an eating disorder, or other health problems; while a BMI greater than 25 is considered overweight and above 30 is considered obese.
There are differing opinions on the threshold for being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI <17.5 being an informal criterion for the diagnosis of anorexia nervosa.
These ranges of BMI values are valid only as statistical categories when applied to adults, and do not predict health:
BMI range – kg/m2
Mass (weight) of a 1.8 metres (5 ft 11 in) person with this BMI
less than 16.5
less than 0.66
under 53.5 kilograms (8.42 st; 118 lb)
from 16.5 to 18.5
from 0.66 to 0.74
between 53.5 and 60 kilograms (8.42 and 9.45 st; 118 and 132 lb)
from 18.5 to 25
from 0.74 to 1.0
between 60 and 81 kilograms (9.4 and 13 st; 130 and 180 lb)
from 25 to 30
from 1.0 to 1.2
between 81 and 97 kilograms (12.8 and 15.3 st; 180 and 210 lb)
Obese Class I
from 30 to 35
from 1.2 to 1.4
between 97 and 113 kilograms (15.3 and 17.8 st; 210 and 250 lb)
Obese Class II
from 35 to 40
from 1.4 to 1.6
between 113 and 130 kilograms (17.8 and 20.5 st; 250 and 290 lb)
from 40 to 45
from 1.6 to 1.8
between 130 and 146 kilograms (20 and 23 st; 290 and 320 lb)
from 45 to 50
from 1.8 to 2.0
between 146 and 162 kilograms (23.0 and 25.5 st; 320 and 360 lb)
from 50 to 60
from 2.0 to 2.4
between 162 and 194 kilograms (25.5 and 30.5 st; 360 and 430 lb)
above 194 kilograms (30.5 st; 430 lb)
The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more — was found in 2% of the men and 4% of the women. The newest survey in 2007 indicates a continuation of the increase in BMI, 63% of Americans are overweight, with 26% now in the obese category.
The BMI Controversy
As a measure, BMI became popular during the early 1950s and 1960s as obesity started to become a discernible issue in prosperous Western societies. BMI provided a simple numeric measure of a person’s “fatness” or “thinness”, allowing health professionals to discuss over- and under-weight problems more objectively with their patients. However, BMI has become controversial because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI’s purpose. It is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition. For these individuals, the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a BMI below 17.5 may indicate the person has anorexia or a related disorder; a number above 30 suggests the person is obese (over 40, morbidly obese).
BMI can be calculated quickly and without expensive equipment. However, BMI categories do not take into account many factors such as frame size and muscularity. The categories also fail to account for varying proportions of fat, bone, cartilage, water weight, and more.
One basic problem, especially in athletes, is that muscle is denser than fat. Some professional athletes are “overweight” or “obese” according to their BMI – unless the number at which they are considered “overweight” or “obese” is adjusted upward in some modified version of the calculation. In children and the elderly, differences in bone density and, thus, in the proportion of bone to total weight can mean the number at which these people are considered underweight should be adjusted downward.
For a fixed body shape and body density, and given height, BMI is proportional to weight. However, for a fixed body shape and body density, and given weight, BMI is inversely proportional to the square of the height. So, if all body dimensions double, and weight scales naturally with the cube of the height, then BMI doubles instead of remaining the same. This results in taller people having a reported BMI that is uncharacteristically high compared to their actual body fat levels. This anomaly is partially offset by the fact that many taller people are not just “scaled up” short people, but tend to have narrower frames in proportion to their height. It has been suggested that instead of squaring the body height (as the BMI does) or cubing the body height (as seems natural), it would be more appropriate to use an exponent of between 2.3 to 2.7.
The medical establishment has generally acknowledged some shortcomings of BMI. Because the BMI is dependent only upon weight and height, it makes simplistic assumptions about distribution of muscle and bone mass, and thus may overestimate adiposity on those with more lean body mass (e.g. athletes) while underestimating adiposity on those with less lean body mass (e.g. the elderly).
One recent study Romero-Corral et al. found that BMI-defined obesity was present in 19.1% of men and 24.7% of women, but that obesity as measured by bodyfat percentage was present in 43.9% of men and 52.3% of women. Moreover, in the intermediate range of BMI (25-29.9), BMI failed to discriminate between bodyfat percentage and lean mass. The study concluded that “the accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly… These results may help to explain the unexpected better survival in overweight/mild obese patients.”
The exponent of 2 in the denominator of the formula for BMI is arbitrary. It is meant to reduce variability in the BMI associated only with a difference in size, rather than with differences in weight relative to one’s ideal weight. If taller people were simply scaled-up versions of shorter people, the appropriate exponent would be 3, as weight would increase with the cube of height. However, on average, taller people have a slimmer build relative to their height than do shorter people, and the exponent which matches the variation best is between 2 and 3. An analysis based on data gathered in the USA suggested an exponent of 2.6 would yield the best fit for children aged 2 to 19 years old. The exponent 2 is used instead by convention and for simplicity.
Some argue that the error in the BMI is significant and so pervasive that it is not generally useful in evaluation of health. Owing to these limitations, body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skinfold measurements or underwater weighing and the limitations of manual measurement have also led to new, alternative methods to measure obesity, such as the body volume index. However, recent studies of American football linemen who undergo intensive weight training to increase their muscle mass show that they frequently suffer many of the same problems as people ordinarily considered obese, notably sleep apnea.
In an analysis of 40 studies involving 250,000 people, heart patients with normal BMIs were at higher risk of death from cardiovascular disease than people whose BMIs put them in the “overweight” range (BMI 25-29.9). Patients who were underweight (BMI <20) or severely obese (BMI >35) did, however, show an increased risk of death from cardiovascular disease. The implications of this finding can be confounded by the fact that many chronic diseases, such as diabetes, can cause weight loss before the eventual death. In light of this, higher death rates among thinner people would be the expected result.
A further limitation relates to loss of height through aging. In this situation, BMI will increase without any corresponding increase in weight.
To overcome the shortcomings of BMI, and some of the less acknowledged limitations inherent in body fat percentages, the concepts fat-free mass index (FFMI) and fat mass index (FMI) were introduced in the early 1990s (VanItallie TB, Yang MU, Heymsfield SB, Funk RC, Boileau RA. Height-normalized indices of the body’s fat-free mass and fat mass: potentially useful indicators of nutritional status. Am J Clin Nutr. Dec 1990;52(6):953-959).