Why is BMI used to measure obesity



The abbreviation BMI stands for Body Mass Index ("body mass index") and is an internationally recognized unit of measurement with which the individual body weight can be assessed. The BMI represents the body weight of a person in relation to the body size and serves as a rough classification of the nutritional status. [1]

Formula for calculating the BMI

BMI = body weight (in kilograms): height² (in meters) [1]

BMI classification for adults

The following table can be used to determine in which range your own body weight is:




BMI (kg / m2)

Underweight <18,5
Normal weight 18,5 - 24,9
Obesity > 25,0
Pre-obesity 25 - 29,9
Adipostitas I 30 - 34,9
Obesity II 35 - 39,9
Obesity III ≥ 40

Table 1: Classification according to WHO 2000 [2]

Example of a BMI calculator for adults [7]

According to the Austrian Nutrition Report 2017, 41% of the examined adult population (18 to under 65 years of age) are overweight or obese. Men are more likely to be overweight than women. The calculated body mass index (BMI) of the participants averages 25.0 kg / m²; Women have a mean BMI of 24.2 kg / m², men 25.8 kg / m². [9]

A high BMI (overweight or obesity) can lead to serious nutrition-related diseases. The health consequences are, for example, an increased risk of non-communicable diseases (noncommunicable diseases, NCDs), such as cardiovascular diseases, type 2 diabetes mellitus, musculoskeletal disorders, and various cancers. [9]

BMI classification for children and adolescents

As with adults, the BMI can also be used for children and adolescents to assess overweight and obesity, but different values ​​apply to this group of people. Since the BMI in childhood and adolescence is influenced by clear age and gender-specific characteristics in accordance with the physiological changes in the percentage of body fat mass, this must be taken into account in the assessment. The age- and gender-specific percentile curves according to Kromeyer-Hausschild et al. (2001) used. The determination of overweight or obesity and extreme obesity should be based on the 90th or 97th and 99.5. Percentiles of this reference data. [1,3,5]


BMI percentiles

Obesity BMI percentiles> 90 - 97
Obesity BMI percentiles> 97 - 99.5
extreme obesity BMI percentiles> 99.5

Table 2: BMI classification for children and adolescents [3]

Example of a BMI calculator for children and adolescents [8]

BMI classification for the elderly

With increasing age, the desirable BMI is higher than that of younger people. Older people with a lower BMI are more susceptible to certain diseases. Energy reserves can reduce the risk of mortality in the event of illness. [6]

Therefore, a different BMI assessment applies to people over the age of 65:


BMI (kg / m²)

Severe malnutrition < 18,5
Slight malnutrition 18,5 - 19,99
Risk of malnutrition 20 - 21,99
Normal weight 22 - 26,99
Preadipose 27-29,99
Obesity ≥ 30

Table 3: BMI classification for people over 65 years of age [6]

Limits of the BMI

The classification by means of the body mass index serves as a rough classification of the nutritional status. The relationship between body size and body weight is not sufficient to make statements about body composition. [1]

The BMI does not differentiate between metabolically active cell mass and fat mass. People with high muscle mass but rather low or “normal” fat mass are classified as overweight according to their BMI.

  • BMI and height:

The meaningfulness of the BMI is limited for very tall / very small people. The traditional calculation of the BMI can be improved with the help of a 3D body scan, for example, since the body surface is included here. [1]

  • BMI and body fat percentage:

According to the surveys of the Austrian Nutrition Report 2017, with regard to obesity prevalence in 96.8% of the cases there is a correspondence between the classification according to the BMI categories and the risk assessment based on the body fat percentage. In the normal weight range, the BMI underestimated the body fat percentage in 25.7% of the collective and overestimated it in 11.8%. In the underweight group, the BMI most often classifies body weight (38.7%) as lower than it corresponds to the classification according to the percentage of body fat. [9]

Further anthropometric parameters

When considering the waist circumference or the waist-to-hip ratio, the results are more meaningful. [4]

1.      Waist circumference (= waist circumference at the level of the navel)

Limit for an increased cardiovascular risk [4]:

in women Waist circumference up to 80 cm
in men Waist circumference up to 102 cm

Increased risk of cardiovascular diseases [4]:

in women Waist circumference> 88 cm
in men Waist circumference> 102 cm

According to the Austrian Nutrition Report 2017, the average waist circumference of the study collective was 86.5 cm. The mean waist circumference is 81.0 cm for women and 91.9 cm for men. [9]

2.      Waist-to-hip ratio (WHR, waist circumference / hip circumference)

Not only the mass and percentage of fat in the human body affect health, but also the distribution of adipose tissue. By comparing waist and hip measurements, a distinction can be made between Android (increased fat accumulation above the waist, "apple type") and gynoid (increased fat accumulation below the waist, "pear type") fat distribution. The android fat distribution increases the risk of cardiovascular disease because it is associated with a larger amount of metabolically active visceral fat. [4]

Significantly increased risk of cardiovascular diseases according to WHO [4]:

in women WHR> 0.85
in men WHR> 0.9

The average waist-to-hip ratio for all participants was calculated to be 0.85. Women have a WHR of 0.80 and men of 0.89. [9]

Other methods are used to precisely measure body composition (see “Measuring body composition”).


© ÖGE (2019)

[1] ELMADFA I, LEITZMANN C. Human nutrition. 5th edition. Verlag Eugen Ulmer Stuttgart 2015. Chapter 2.1. Body composition (page 37 ff.)

[2] WHO. Obesity: preventing and managing the global epidemic. Report of a WHO Consultation. WHO Technical Report Series 894, Geneva. 2000.

[3] WABITSCH M, KUNZE D. (in charge of the AGA). Consensus-based guideline for the diagnosis, therapy and prevention of overweight and obesity in children and adolescents. Version 2015. Internet: https://aga.adipositas-gesellschaft.de/fileadmin/PDF/Leitlinien/AGA_S2_Leitlinie.pdf (Access: August 21, 2019)

[4] NHLBI. Obesity Education Initiative. The practical guide: Identification, evaluation and treatment of overweight and obesity in adults. National Institutes of Health. 2000 (NIH Publication Number 00-4084). Internet: https://www.nhlbi.nih.gov/files/docs/guidelines/prctgd_c.pdf (Access: August 21, 2019)

[5] KROMEYER-HAUSCHILD K., et al. Percentiles for the body mass index for children and adolescents using various German samples. 2001. Monthly Pediatric Medicine 149 (8): 807-818

[6] VOLKERT D, BERNER YN, BERRY E, CEDERHOLM T, COTI BERTRAND P, MILNE A, PALMBLAD J, SCHNEIDER S, SOBOTKA L, STANGA Z, et al, ESPEN Guidelines on Enteral Nutrition: Geriatrics. Clin Nutr. 2006; 25 (2): 330-60

[7] BMI CALCULATOR: GERMAN OBESITY GESELLSCHAFT. What is the BMI. Internet: https://www.adipositas-gesellschaft.de/mybmi/ (Access: August 21, 2019)

[8] BMI-CALCULATOR: WORKING COMMUNITY OBESITY IN CHILDHOOD AND YOUTH AGE. What is the BMI. Internet: https://aga.adipositas-gesellschaft.de/mybmi4kids/ (Access: August 21, 2019)

[9] RUST P, HASENEGGER V, KÖNIG J. Austrian Nutrition Report 2017. 1st edition, Vienna, 2017. Chapter body weight and body composition in Austrian adults.