About ideal weight (IW) and body mass index (BMI)
«How much do I want to lose?» – the answer often rests on a number from years ago. Other questions are more useful: since when has my weight been above the range in which I feel comfortable? What have I tried since then, and what has stayed?
«Ideal weight» is a guide value, not a goal
Historical formulas such as those of Lorentz or the Metropolitan Life Insurance calculate a theoretical weight from height, sex and age. They say nothing about how much of it is muscle, bone or water – and therefore little about health. Today it is mainly the body mass index that serves as orientation.
Body mass index (BMI)
BMI = body weight in kilograms divided by height in metres squared.
In adults the normal range lies between 18,5 and 24,9 kg/m². For a person of 1,63 m that works out at roughly 49 to 66 kg. From 25 kg/m² one speaks of overweight, from 30 kg/m² of obesity. The World Health Organization recommends the BMI as a simple measure at population level.
| BMI (kg/m²) | Classification |
|---|---|
| under 18,5 | Underweight |
| 18,5 – 24,9 | Normal range |
| 25 – 29,9 | Overweight |
| 30 and above | Obesity |
Where the BMI reaches its limits
The BMI does not distinguish between fat and muscle mass. Athletes therefore reach high values without carrying fat reserves; in older people the opposite can be true. Water retention also distorts the picture.
More telling in addition is the waist circumference, because fat in the abdomen is linked more strongly to metabolic and cardiovascular risks than fat on hips and legs. As a guide, more than 94 cm in men and more than 80 cm in women is a sign to stop the weight gain; from 102 cm and 88 cm respectively, a reduction is advisable. Measurement is taken standing, halfway between the lowest rib and the iliac crest.
Pace counts for more than the number
An unintended loss of around ten per cent of body weight is a reason to have it checked medically – as is a rapid gain. For planned weight loss: about 0,5 kg a week is realistic and easy to hold; faster usually means that mostly water and muscle mass are lost.
Different rules apply for children
Because height and weight do not increase in parallel during growth, in children and adolescents the BMI is always related to growth charts and assessed by a professional.
This is general information. It does not replace a medical assessment – especially with illnesses, during pregnancy and with marked under- or overweight.