What is BMI?
Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from mass in kilograms and height in metres.
The BMI may be determined using a table or chart which displays BMI as a function of mass and height using contour lines or colours for different BMI categories, and which may use other units of measurement (converted to metric units for the calculation).
The BMI is a convenient rule of thumb used to broadly categorize a person as underweight, normal weight, overweight, or obese based on tissue mass (muscle, fat, and bone) and height. Commonly accepted BMI ranges are underweight (under 18.5 kg/m2), normal weight (18.5 to 25), overweight (25 to 30), and obese (over 30).
BMIs under 20 and over 25 have been associated with higher all-causes mortality, with the risk increasing with distance from the 20–25 range.
BMI is proportional to the mass and inversely proportional to the square of the height. So, if all body dimensions double, and mass 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. In comparison, the Ponderal index is based on the natural scaling of mass with the third power of the height.
However, many taller people are not just "scaled up" short people but tend to have narrower frames in proportion to their height. Carl Lavie has written that, "The B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals."
A common use of the BMI is to assess how far an individual's body weight departs from what is normal or desirable 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 a BMI of less than 18.5 as underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI equal to or greater than 25 is considered overweight and above 30 is considered obese. These ranges of BMI values are valid only as statistical categories.
Very severely underweight
Normal (healthy weight)
Obese Class I (Moderately obese)
Obese Class II (Severely obese)
Obese Class III (Very severely obese)
Children (aged 2 to 20)
BMI is used differently for children. It is calculated in the same way as for adults, but then compared to typical values for other children 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.
Recent studies in Britain have indicated that females between the ages 12 and 16 have a higher BMI than males of the same age by 1.0 kg/m2 on average.
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 ethnic groups, 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.
The Hospital Authority of Hong Kong recommends the use of the following BMI ranges:
In Singapore, the BMI cut-off figures were revised in 2005, 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.
Risk of developing problems such as nutritional deficiency and osteoporosis
Low Risk (healthy range)
18.5 to 23
Moderate risk of developing heart disease, high blood pressure, stroke, diabetes
23 to 27.5
High risk of developing heart disease, high blood pressure, stroke, diabetes
In 1998, the U.S. National Institutes of Health and the Centers for Disease Control and Prevention brought U.S. definitions in line with World Health Organization guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 29 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 South East 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 for males and females aged 20 and over, and selected percentiles by age: United States, 2011–2014.
5th 10th 15th 25th 50th 75th 85th 90th 95th
Men BMI (kg/m2)
20 years and over (total) 20.7 22.2 23.0 24.6 27.7 31.6 34.0 36.1 39.8
20–29 years 19.3 20.5 21.2 22.5 25.5 30.5 33.1 35.1 39.2
30–39 years 21.1 22.4 23.3 24.8 27.5 31.9 35.1 36.5 39.3
40–49 years 21.9 23.4 24.3 25.7 28.5 31.9 34.4 36.5 40.0
50–59 years 21.6 22.7 23.6 25.4 28.3 32.0 34.0 35.2 40.3
60–69 years 21.6 22.7 23.6 25.3 28.0 32.4 35.3 36.9 41.2
70–79 years 21.5 23.2 23.9 25.4 27.8 30.9 33.1 34.9 38.9
80 years and over 20.0 21.5 22.5 24.1 26.3 29.0 31.1 32.3 33.8
Age Women BMI (kg/m2)
20 years and over (total) 19.6 21.0 22.0 23.6 27.7 33.2 36.5 39.3 43.3
20–29 years 18.6 19.8 20.7 21.9 25.6 31.8 36.0 38.9 42.0
30–39 years 19.8 21.1 22.0 23.3 27.6 33.1 36.6 40.0 44.7
40–49 years 20.0 21.5 22.5 23.7 28.1 33.4 37.0 39.6 44.5
50–59 years 19.9 21.5 22.2 24.5 28.6 34.4 38.3 40.7 45.2
60–69 years 20.0 21.7 23.0 24.5 28.9 33.4 36.1 38.7 41.8
70–79 years 20.5 22.1 22.9 24.6 28.3 33.4 36.5 39.1 42.9
80 years and over 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
- Type 2 diabetes
- Gallbladder disease
- Sleep apnea
At least 10 cancers, including endometrial, breast, and colon cancer.
Among people who have never smoked, overweight/obesity is associated with 51% increase in mortality compared with people who have always been a normal weight.