Body mass index (BMI)
BMI relates weight to height – a rough guide that needs to be read differently for children, athletes and older people.
Ausführlich erklärt
Body mass index (BMI) describes the ratio of body weight to height – it's the best-known guide to whether a weight is in the healthy range. The formula: weight in kilograms divided by height in metres squared. If you're 1.70 m tall and weigh 68 kg, you calculate 68 ÷ (1.7 × 1.7) and end up with a BMI of about 23.5.
The World Health Organization (WHO) classifies adults as follows: below 18.5 underweight, 18.5 to 24.9 normal weight, 25 to 29.9 overweight, 30 and above obesity. These cut-offs apply equally to women and men and are based on large population studies: from a BMI of 25, the risk of type 2 diabetes, high blood pressure and joint problems rises on average.
But BMI has clear limitations. It doesn't distinguish between muscle and fat: a well-trained handball player can have a BMI of 27 and still be very lean. For people over 65, a slightly higher value is often considered harmless, and BMI doesn't apply during pregnancy. It also says nothing about where fat is stored – yet belly fat matters more for health than fat on the hips and legs. That's why it's worth also looking at your waist-to-height ratio (WHtR).
The adult cut-offs don't apply to children and teenagers. Because body build and body fat change constantly during growth, their BMI is compared with percentile curves for age and sex – a percentile tells you what percentage of children of the same age have a lower value. In Germany, a BMI above the 90th percentile counts as overweight and above the 97th as obesity – this is best assessed by the paediatrician at a routine check-up.
Our tip: take BMI as a first indication, not a verdict. How you feel, how fit you are and what your waist measurement looks like say at least as much.