Body mass index (BMI) is a value derived from the mass (weight) and height of a person. BMI is calculated as the body mass, in kilograms (kg), divided by the square of the body height, in square metres (m2). Although this quotient has units of kilograms per square metre, BMI is frequently reported with these units omitted (despite not being a dimensionless quantity).
The BMI is a convenient rule of thumb used to broadly categorize a person based on tissue mass (muscle, fat, and bone) and height. Major adult BMI classifications are: underweight (under 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or more). When used to predict an individual's health, rather than as a statistical measurement for groups, the BMI has limitations that can make it less useful than some of the alternatives, especially when applied to individuals with abdominal obesity, short stature, or high muscle mass.
BMI values under 20 and over 25 have been associated with higher all-cause mortality, with the risk increasing with distance from the 20–25 interval.
History
Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist, devised the basis of the BMI between 1830 and 1850 as he developed what he called "social physics". Quetelet himself never intended for the index, then called the Quetelet Index, to be used as a means of medical assessment. Instead, it was a component of his study of , or the average man. Quetelet thought of the average man as a social ideal, and developed the body mass index as a means of discovering the socially ideal human person. According to Lars Grue and Arvid Heiberg in the Scandinavian Journal of Disability Research, Quetelet's idealization of the average man would be elaborated upon by Francis Galton a decade later in the development of eugenics.
The modern term "body mass index" (BMI) for the ratio of human body weight to squared height was coined in a paper published in the July 1972 edition of the Journal of Chronic Diseases by Ancel Keys and others. In this paper, Keys argued that what he termed the BMI was "if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity".
The interest in an index that measures body fat came with observed increasing obesity in prosperous Western societies. Keys explicitly judged BMI as appropriate for population studies and inappropriate for individual evaluation. This distinction continues to be cited as a limitation of BMI because individuals with the same BMI can differ substantially in body composition and fat distribution. Nevertheless, due to its simplicity, it has come to be widely used for preliminary diagnoses. Additional metrics, such as waist circumference, can be more useful.
Calculation
The BMI is expressed in kg/m2, resulting from mass in kilograms and height in metres. If pounds and inches are used, a conversion factor of 703 (kg/m2)/(lb/in2) is applied. (If pounds and feet are used, a conversion factor of 4.88 is used.) When the term BMI is used informally, the units are usually omitted.
BMI provides a simple numeric measure of a person's thickness or thinness, allowing health professionals to discuss weight problems more objectively with their patients. BMI was designed to be used as a simple means of classifying average sedentary (physically inactive) populations, with an average body composition. For such individuals, the BMI value recommendations are as follows: 18.5 to 24.9 kg/m2 may indicate optimal weight, lower than 18.5 may indicate underweight, 25 to 29.9 may indicate overweight, and 30 or more may indicate obese. Lean male athletes often have a high muscle-to-fat ratio and therefore a BMI that is misleadingly high relative to their body-fat percentage.
The BMI may be determined first by measuring its components by means of a weighing scale and a stadiometer. The multiplication and division may be carried out directly, by hand or using a calculator, or indirectly using a lookup table (or chart). The table displays BMI as a function of mass and height and may show other units of measurement (converted to metric units for the calculation). The table may also show contour lines or colours for different BMI categories.
Categories A common use of the BMI is to assess how far an individual's body weight departs from what is normal for a person's 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 WHO regards an adult BMI of less than 18.5 as underweight and possibly indicative of malnutrition, an eating disorder, or other health problems, while a BMI of 25 or more is considered overweight and 30 or more is considered obese. In addition to the principle, international WHO BMI cut-off points (16, 17, 18.5, 25, 30, 35 and 40), four additional cut-off points for at-risk Asians were identified (23, 27.5, 32.5 and 37.5). These ranges of BMI values are valid only as statistical categories.
+ BMI, basic categories Category BMI (kg/m2) BMI Prime Underweight (Severe thinness) < 16.0 < 0.64 Underweight (Moderate thinness) 16.0–17.0 0.64–0.68 Underweight (Mild thinness) 17.0–18.5 0.68–0.74 Normal range 18.5–25.0 0.74–1.00 Overweight (Pre-obese) 25.0–30.0 1.00–1.20 Obese (Class I) 30.0–35.0 1.20–1.40 Obese (Class II) 35.0–40.0 1.40–1.60 Obese (Class III) ≥ 40.0 ≥ 1.60
Children and youth
BMI is used differently for people aged 2 to 20. It is calculated in the same way as for adults but then compared to typical values for other children or youth of the same age. Instead of comparison against fixed thresholds for underweight and overweight, the BMI is compared against the percentiles for children of the same sex and age.
A BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is considered obese. Children with a BMI between the 85th and 95th percentile are considered to be overweight.
Studies in Britain from 2013 have indicated that females between the ages 12 and 16 had a higher BMI than males of the same age by 1.0 kg/m2 on average.
International variations These recommended distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. People from different populations and descent have different associations between BMI, percentage of body fat, and health risks, with a higher risk of type 2 diabetes mellitus and atherosclerotic cardiovascular disease at BMIs lower than the WHO cut-off point for overweight, 25 kg/m2, although the cut-off for observed risk varies among different populations. The cut-off for observed risk varies based on populations and subpopulations in Europe, Asia and Africa.
Hong Kong The Hospital Authority of Hong Kong recommends the use of the following BMI ranges:
+ BMI in Hong Kong Category BMI (kg/m2) Underweight (Unhealthy) < 18.5 Normal range (Healthy) 18.5–22.9 Overweight I (At risk) 23.0–24.9 Overweight II (Moderately obese) 25.0–29.9 Overweight III (Severely obese) ≥ 30.0
Japan A 2000 study from the Japan Society for the Study of Obesity (JASSO) presents the following table of BMI categories:
+ BMI in Japan Category BMI (kg/m2) Underweight (Thin) < 18.5 Normal weight 18.5–24.9 Obesity (Class 1) 25.0–29.9 Obesity (Class 2) 30.0–34.9 Obesity (Class 3) 35.0–39.9 Obesity (Class 4) ≥ 40.0
Singapore In Singapore, the BMI cut-off figures were revised in 2005 by the Health Promotion Board (HPB), motivated by studies showing that many Asian populations, including Singaporeans, have a higher proportion of body fat and increased risk for cardiovascular diseases and diabetes mellitus, compared with general BMI recommendations in other countries. The BMI cut-offs are presented with an emphasis on health risk rather than weight.
+ BMI in Singapore Category BMI (kg/m2) Health risk Underweight < 18.5 Possible nutritional deficiency and osteoporosis. Normal 18.5–22.9 Low risk (healthy range). Mild to moderate overweight 23.0–27.4 Moderate risk of developing heart disease, high blood pressure, stroke, diabetes mellitus. Very overweight to obese ≥ 27.5 High risk of developing heart disease, high blood pressure, stroke, diabetes mellitus. Metabolic syndrome.
United Kingdom In the UK, NICE guidance recommends prevention of type 2 diabetes should start at a BMI of 30 in White and 27.5 in Black African, African-Caribbean, South Asian, and Chinese populations.
Research since 2021 based on a large sample of almost 1.5 million people in England found that some ethnic groups would benefit from prevention at or above a BMI of (rounded): 30 in White 28 in Black just below 30 in Black British 29 in Black African 27 in Black Other 26 in Black Caribbean 27 in Arab and Chinese 24 in South Asian 24 in Pakistani, Indian and Nepali 23 in Tamil and Sri Lankan 21 in Bangladeshi
United States In 1998, the U.S. National Institutes of Health brought U.S. definitions in line with World Health Organization guidelines, lowering the normal/overweight cut-off from a BMI of 27.8 (men) and 27.3 (women) to a BMI of 25. This had the effect of redefining approximately 25 million Americans, previously healthy, to overweight.
This can partially explain the increase in the overweight diagnosis in the past 20 years, and the increase in sales of weight loss products during the same time. WHO also recommends lowering the normal/overweight threshold for southeast Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.
A survey in 2007 showed 63% of Americans were then overweight or obese, with 26% in the obese category (a BMI of 30 or more). By 2014, 37.7% of adults in the United States were obese, 35.0% of men and 40.4% of women; class 3 obesity (BMI over 40) values were 7.7% for men and 9.9% for women. The U.S. National Health and Nutrition Examination Survey of 2015–2016 showed that 71.6% of American men and women had BMIs over 25. Obesity—a BMI of 30 or more—was found in 39.8% of the US adults.
+ Body mass index values (kg/m2) for males aged 20 and over, and selected percentiles by age: United States, 2011–2014 Age Percentile 5th 10th 15th 25th 50th 75th 85th 90th 95th ≥ 20 (total) 20.7 22.2 23.0 24.6 27.7 31.6 34.0 36.1 39.8 20–29 19.3 20.5 21.2 22.5 25.5 30.5 33.1 35.1 39.2 30–39 21.1 22.4 23.3 24.8 27.5 31.9 35.1 36.5 39.3 40–49 21.9 23.4 24.3 25.7 28.5 31.9 34.4 36.5 40.0 50–59 21.6 22.7 23.6 25.4 28.3 32.0 34.0 35.2 40.3 60–69 21.6 22.7 23.6 25.3 28.0 32.4 35.3 36.9 41.2 70–79 21.5 23.2 23.9 25.4 27.8 30.9 33.1 34.9 38.9 ≥ 80 20.0 21.5 22.5 24.1 26.3 29.0 31.1 32.3 33.8
+ Body mass index values (kg/m2) for females aged 20 and over, and selected percentiles by age: United States, 2011–2014 Age Percentile 5th 10th 15th 25th 50th 75th 85th 90th 95th ≥ 20 (total) 19.6 21.0 22.0 23.6 27.7 33.2 36.5 39.3 43.3 20–29 18.6 19.8 20.7 21.9 25.6 31.8 36.0 38.9 42.0 30–39 19.8 21.1 22.0 23.3 27.6 33.1 36.6 40.0 44.7 40–49 20.0 21.5 22.5 23.7 28.1 33.4 37.0 39.6 44.5 50–59 19.9 21.5 22.2 24.5 28.6 34.4 38.3 40.7 45.2 60–69 20.0 21.7 23.0 24.5 28.9 33.4 36.1 38.7 41.8 70–79 20.5 22.1 22.9 24.6 28.3 33.4 36.5 39.1 42.9 ≥ 80 19.3 20.4 21.3 23.3 26.1 29.7 30.9 32.8 35.2
Consequences of elevated level in adults The BMI ranges are based on the relationship between body weight and disease and death. Overweight and obese individuals are at an increased risk for the following diseases: Coronary artery disease Dyslipidemia Type 2 diabetes Epidural lipomatosis Gallbladder disease Hypertension Infertility Lymphedema, secondary; subsequent to RT and (breast-conserving mastectomy or axillary lymphadenectomy) Osteoarthritis Sleep apnea Stroke At least 10 cancers, including endometrial, breast, and colon cancer