Body Mass Index

Synonyms in a broader sense

BMI, Mass Index, Quetelet-Index overweight, obesity, obesity, body fat

What is the body mass index?

The BMI is a key figure that can be used to assess whether a person is overweight and, if so, how much, and enables a classification. The Body Mass Index is recommended by the World Health Organization (WHO) as a guideline. The BMI does not take into account gender, age and stature and is only valid for adults.

For people under 18 years of age, so-called percentile curves (reference curves) have been drawn up on the basis of the BMI. Here, growth, age and gender were included in the calculation and summarised in tables. These tables are used to assess the body weight of children and adolescents.

Calculation of the BMI

The formula for calculating the BMI in adults goes back to the Belgian mathematician Adolph Quetelet and is: weight/kgBMI = ————————————————————— Height x height The following weight classifications result: Since the given formula ignores both the muscle and fat content of the person in question, a more differentiated consideration of the body composition would be advisable to use, among other things, bioelectric impedance analysis (BIA). By differentiating between fat and muscle tissue, this can provide a higher quality statement about the training and health status of a person than the BMI alone. – Underweight: <18.5

  • Normal weight: 18,5 – 25,0
  • Overweight: >25,0
  • Preadiposity: 25.0 – 30.0
  • Obesity grade I: 30.0 – 35.0
  • Obesity grade II: 35.0 – 40.0
  • Obesity grade III: >40.0


Underweight is defined as having a BMI of less than 18.5. This is initially not a pathological condition, but it does involve health risks and a medical checkup is necessary. A lack of supply of important nutrients can impair health. In this context, an eating disorder must be considered, especially among young people. Serious diseases can also lead to underweight.

Normal weight

According to the BMI (Body Mass Index), the range of normal body weight is between 18.5 and 25.0. This classification is not uncritical. Athletes with increased body weight due to increased muscle mass (muscles weigh more than fat tissue) and “overweight BMI” should additionally measure the abdominal girth and determine the body fat percentage.

The BMI (Body Mass Index) only takes into account body weight and height, but not the body composition. In general, well-trained people weigh more than untrained people because of the increased muscle mass. The so-called normal weight also increases with age.

While the optimal BMI for 19 to 24-year-olds is between 19 and 24, the “green range” for 45-54-year-olds shifts to 22 to 27 and seniors over 65 years of age may have a BMI of 24 to 29. Since men generally have more muscle mass than women, the German Nutrition Society (DGE) also differentiates according to gender. For example, a BMI of 20 to 25 is quite normal for young men.

A BMI (body mass index) of 25.0 to 30.0 is generally referred to as overweight or, in medical parlance, as preadiposity. However, the criteria already mentioned above, such as age, gender, training condition and risk factors should be taken into account in the assessment. The decision as to whether weight reduction is necessary should be based primarily on the presence of certain risk factors such as high blood pressure, elevated blood sugar, elevated blood lipid levels, elevated uric acid levels or joint problems.

People with a BMI over 25 and under 30 are usually not yet exposed to the risks mentioned. However, there is a chance that they will one day have a BMI above 30. This assumption is based on the fact that people in industrialised countries gain an average of about 5 kg over a period of 10 years.