BMI: meaning, calculation, limits and interpretation
BMI (Body Mass Index) is the most common measure to assess whether your weight is proportional to your height. Simple to calculate, but with important limitations to know. This guide explains everything you need to interpret it correctly.
The BMI formula
BMI is calculated by dividing weight in kilograms by the square of height in meters:
BMI = weight (kg) / height² (m²)
For example, a 70 kg person who is 1.75 m tall has BMI = 70 / (1.75 × 1.75) = 22.9, which falls in the "normal" range.
Standard BMI categories
- Underweight: BMI < 18.5
- Normal: BMI 18.5 — 24.9
- Overweight: BMI 25 — 29.9
- Obese I: BMI 30 — 34.9
- Obese II: BMI 35 — 39.9
- Obese III: BMI ≥ 40
BMI limitations: when it's not reliable
BMI doesn't distinguish between lean mass and fat mass. This means:
- Athletes and bodybuilders may appear "overweight" despite having little fat (muscle weighs more than fat).
- Elderly people may have "normal" BMI but reduced muscle and bone mass (sarcopenia).
- Pregnant women shouldn't use BMI.
- It doesn't account for fat distribution: abdominal fat is more dangerous than fat on hips and thighs.
For a more complete body composition analysis there are more precise measurements: skinfold calipers, bioelectrical impedance (BIA), DEXA. But for most people BMI is a good starting point.
Ideal BMI for longevity
Recent studies suggest the BMI associated with greater longevity is slightly higher than the "normal" range (around 23-25), especially in the elderly. Don't chase a perfect BMI: aim for a healthy lifestyle with balanced nutrition and regular physical activity.
BMI and children
For children and adolescents (under 18) adult ranges are not used, but percentiles relative to peers of the same sex and age. A BMI of 20 in a 10-year-old can indicate overweight, while in an adult it's normal.