Your body mass index (BMI) is the ratio between your height and your body weight. This guide value acts as a pointer for you in terms of preventative healthcare. You can find out here what values are normal and when a person is considered to be under- or overweight.

Calculate your BMI

Please specify a valid value for your weight.
Please specify a valid value for your BMI.
Please specify a valid value for your height.

What does "BMI" mean?

The body mass index (BMI) is a statistical figure indicating the ratio of height to weight. Calculating the BMI is a quick and simple way of assessing a person's weight based on guide values.

The BMI acts as a rough guide for the ideal weight of an individual. It does not, however, take account of their build or the ratio of fat to muscle. It is a standard value and a quick and easy way of assessing a person's weight, so avoiding a health risk from being under- or overweight.

The BMI cannot be applied to children, older persons or people who do a lot of sport as their body mass differs from average values.

What does the BMI indicate?

The body mass index (BMI) defines the ratio between height and weight. This value indicates whether a person falls within the normal range or is under- or overweight.

The BMI is a guide value in health terms, but it disregards certain aspects, e.g. how the body mass is made up in each case (fat, muscle, bone density, etc.).

The BMI formula

The body mass index is based on a simple mathematical formula: It is calculated by taking the body weight in kilograms and dividing it by the square of the height in metres:

BMI = weight (kg) / height (m)2

Example of a calculation

A man is 1.85 metres tall and weighs 90 kilograms. This is how his BMI is calculated: 90 (kg) divided by 1.85 (m)2; the result is 26.3.

On the other hand, a woman 1.63 metres tall and weighing 53 kilograms has a BMI of 19.9.

The age of a person also influences the assessment of their BMI as body weight generally increases as they become older. For this reason, different assessment tables listing other values for the individual age groups are used for adults.

Evaluate BMI result

Tables drawn up by the World Health Organization (WHO) are used to evaluate the body mass index. They list different guide values depending on age.

Each BMI table always shows four main categories:

  • Underweight
  • Normal range
  • Overweight
  • Obese (i.e. excessive weight)

These categories are then broken down into different degrees and levels of severity. A total of eleven weight classes can be shown here.

Identifying you are overweight

Not every pound of excess weight will make you ill, but permanently being overweight increases the risk of numerous diseases including osteoarthritis, arteriosclerosis, high blood pressure, diabetes and various types of cancer. It can lead to breathing difficulties and joint pain, in addition to disorders of the metabolism and sex hormones.

Identifying you are underweight

If your BMI is below 18.5, you are underweight. As the body is at risk of deficiency here, such persons have an increased risk of cardiovascular disease and osteoporosis. The body's metabolism and immune system may also be impaired.

The BMI helps to identify all such risks to health in good time and reduce them. It allows people at risk to document their progress towards an ideal weight – for example, by getting more exercise and changing their diet.

BMI table*

BMI Weight category
Under 18.5
18.5 - 24.9 Normal range
25.0 - 29.9 Pre-obese 
30.0 - 34.9 Obese class 1
35.0 -39.9 Obese class 2
Over 40
Obese class 3

* Source: World Health Organization Europe


Optimum BMI by age

Assessment of the body mass index also depends on the age of the individual. As the bodies of children and adolescents develop neither uniformly or in synchrony, the BMI cannot be used for young persons.

As people age, their proportion of body fat generally increases, which is why different guide values apply to the various age groups. With older people, the BMI is not applicable as no average ratio can be set between height and weight due to the personal development of each individual.

BMI table by age

Age Optimum BMI

* Source: National Research Council

The BMI cannot be used to assess the health of sports people either, as the ratio of height to weight is distorted by their above-average muscle mass. In this case, the proportion of body fat is a more meaningful guide.