What is BMI (Body Mass Index)?
BMI (Body Mass Index) evaluates your body weight relative to your height. Invented in the 19th century by Belgian statistician Adolphe Quetelet, it's now the WHO's international reference.
BMI is calculated by dividing weight (in kg) by the square of height (in meters). It's a simple and quick screening tool, but has important limitations to be aware of.
Calculation Formula
IMC = Poids (kg) / Taille² (m)Example: For a person 1.75m tall weighing 70 kg: BMI = 70 / (1.75 × 1.75) = 22.9 kg/m²
WHO Categories
The World Health Organization has defined thresholds to classify BMI into categories. These thresholds are identical for adult men and women.
BMI Limitations: Why It Doesn't Tell Everything
BMI is a useful but imperfect tool. Here are its main limitations:
- Muscle mass: An athlete or very muscular person can have a high BMI without excess fat. BMI doesn't distinguish muscle from fat.
- Age: The thresholds aren't adapted for children, adolescents, and elderly. Body composition changes with age even at constant BMI.
- Fat distribution: BMI says nothing about where fat is located. However, abdominal (visceral) fat is more dangerous than subcutaneous fat.
- Ethnicity: Health risks associated with the same BMI vary across populations. WHO thresholds were established on Western populations.
Alternatives and Complements to BMI
For a more complete health assessment, combine BMI with other measures:
Waist Circumference
An excellent indicator of cardiovascular risk. Increased risk above 94 cm (men) or 80 cm (women). High risk above 102 cm (men) or 88 cm (women).
Body Fat Percentage
Measurable by bioimpedance or skinfold, it gives a more accurate picture of your body composition. Normal range: 10-20% (men), 18-28% (women).
Waist-to-Hip Ratio
This ratio evaluates fat distribution. A ratio above 0.90 (men) or 0.85 (women) indicates risky abdominal accumulation.
How to Use BMI Correctly
BMI remains a useful screening tool when used correctly. It helps quickly identify at-risk situations and track body weight evolution over time.
Here's how to use it wisely:
- Consider it as one indicator among others, not a verdict
- Combine it with waist circumference, especially if you're athletic
- Track its evolution over time rather than its absolute value
- When in doubt, consult a healthcare professional for a complete assessment