The BMI formula
The body mass index puts your weight in relation to your height: BMI = weight in kg ÷ (height in m)². An example: at 75 kg and 1.80 m you calculate 75 ÷ (1.80 × 1.80) = 23.1. That puts you in the normal weight range. The formula dates back to the 19th century and the Belgian mathematician Adolphe Quetelet, and to this day it is the most widely used reference value for classifying body weight.
BMI table for adults (WHO)
The World Health Organization classifies adult BMI into these categories:
| BMI (kg/m²) | Classification |
|---|---|
| below 16.0 | Severe underweight |
| 16.0 to 16.9 | Moderate underweight |
| 17.0 to 18.4 | Mild underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight (pre-obesity) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
BMI by age
Body composition changes with age, and slightly higher values are considered favorable. The American National Research Council has published age-dependent ranges that are widely used for orientation:
| Age | Favorable BMI range |
|---|---|
| 19 to 24 years | 19 to 24 |
| 25 to 34 years | 20 to 25 |
| 35 to 44 years | 21 to 26 |
| 45 to 54 years | 22 to 27 |
| 55 to 64 years | 23 to 28 |
| 65 years and older | 24 to 29 |
What BMI cannot tell you
BMI is deliberately simple, and that is exactly where its limits are:
- Muscle mass: Muscle weighs more than fat. Strength athletes quickly land in the overweight range without having too much body fat.
- Fat distribution: Abdominal fat matters more for health than fat on hips or legs. BMI does not see this difference.
- Age and sex: The standard categories apply to middle-aged adults. Milder ranges apply to older adults, see the age table.
- Children and teenagers: Percentile curves apply here instead of fixed cut-offs. The assessment belongs to a pediatrician.
- Pregnancy: Gaining weight is part of it and intended, BMI does not apply.
The better quick check: waist to height
The waist-to-height ratio (WHtR) takes abdominal fat into account and studies suggest it is a more meaningful quick indicator of health risks than BMI. The rule of thumb: your waist circumference should be less than half your height. Measure the circumference in the morning, midway between your lowest rib and the top of your hip bone (roughly at navel height), without pulling in your stomach.
BMI for children and teenagers
Fixed BMI cut-offs do not apply to children and teenagers. Their BMI is compared with age- and sex-specific reference curves (percentiles). A child above the 90th percentile is considered overweight, above the 97th obese. For this we have our dedicated BMI calculator for children and teenagers with exactly these reference values. The reliable assessment, especially over time, remains the job of your pediatrician, for example during regular check-ups.
Elevated BMI: sensible next steps
A single measurement is only a snapshot. If your BMI stays above 25 and you want to change something, three approaches have proven useful: realistic eating habits instead of crash diets, more everyday movement and professional support. In our nutrition basics you find clear knowledge without diet myths, our healthy recipes deliver ideas for everyday cooking, and through the coach directory you find support near you or online. If you are clearly overweight or underweight, your doctor should also be involved. How many calories you need per day is shown by our calorie calculator.