Body mass index (BMI) is a value derived from the  () and  of a person. The BMI is defined as the  divided by the  of the , and is universally expressed in  of kg/m2, resulting from mass in  and height in .

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  used to broadly categorize a person as underweight, normal weight, overweight, or obese based on tissue mass (, fat, and bone) and height. That categorization is the subject of some debate about where on the BMI scale the dividing lines between categories should be placed. 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).

A graph of body mass index as a function of body mass and body height. The dashed lines represent subdivisions within a major class.

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.

, 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". The modern term "body mass index" (BMI) for the ratio of  to squared height was coined in a paper published in the July 1972 edition of the  by  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  came with observed increasing obesity in prosperous  societies. Keys explicitly judged BMI as appropriate for population studies and inappropriate for individual evaluation. 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.

The BMI is universally 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) must be applied. When the term BMI is used informally, the units are usually omitted.

Obesity and BMI


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 . For such individuals, the value recommendations as of 2014 are as follows: a BMI from 18.5 up to 25 kg/m2 may indicate optimal weight, a BMI lower than 18.5 suggests the person is , a number from 25 up to 30 may indicate the person is , and a number from 30 upwards suggests the person is . 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.


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  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 () although other factors such as muscularity also affect BMI significantly (see discussion below and ).

The  regards a BMI of less than 18.5 as underweight and may indicate , an , or other health problems, while a BMI equal to or greater than 25 is considered overweight and above 30 is considered . These ranges of BMI values are valid only as statistical categories.

Category BMI (kg/m2) BMI Prime
from to from to
Very severely underweight   15   0.60
Severely underweight 15 16 0.60 0.64
Underweight 16 18.5 0.64 0.74
Normal (healthy weight) 18.5 25 0.74 1.0
Overweight 25 30 1.0 1.2
Obese Class I (Moderately obese) 30 35 1.2 1.4
Obese Class II (Severely obese) 35 40 1.4 1.6
Obese Class III (Very severely obese) 40   1.6  

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  for children of the same sex and age.
BMI for age percentiles for boys 2 to 20 years of 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.

BMI for age percentiles for girls 2 to 20 years of age.

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.

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 ethnic groups, populations, and descent have different associations between BMI, percentage of body fat, and health risks, with a higher risk of  and  cardiovascular disease at BMIs lower than the  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 both in Europe and Asia.

United States[]

In 1998, the U.S.  and the  brought U.S. definitions in line with  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.  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.

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. A survey in 2007 showed 63% of Americans are overweight or obese, with 26% in the obese category (a BMI of 30 or more). As of 2014, 37.7% of adults in the United States were obese, categorized as 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.

Body Mass Index values for males and 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
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