Calculating BMI

Body mass index (weight/height²). The first health benchmark, to be completed with other measurements (fat mass, waist circumference).

What is BMI?

The BMI (body mass index, or BMI in English) is a reference indicator created in the 19th century by the Belgian statistician Adolphe Quetelet. Formula: BMI = weight (kg) / height (m)². This is the first weight health benchmark, validated by the WHO in 1997.

WHO adult classification

< 16.5: undernutrition
16.5 – 18.5: thinness
18.5 – 25: normal build
25 – 30: overweight
30 – 35: moderate obesity (class I)
35 – 40: severe obesity (class II)
≥ 40: morbid obesity (class III)

The limits of BMI

BMI is an excellent statistical benchmark, but it has individual limits:

Muscular athlete: can have a BMI ≥ 25 without excess fat. Combine with fat mass rate.
Elderly person: muscle mass decreases, BMI may underestimate risk.
Pregnant or breastfeeding woman: distorted interpretation, do not use.
Children/teenagers: use growth curves, not the 18.5–25 adult threshold.

Waistline: the essential complement

Waist circumference is an excellent independent predictor of cardio-metabolic risk. Measured halfway between the bottom of the ribs and the iliac crest:

Men: < 94 cm normal · 94–101 cm to monitor · ≥ 102 cm high
Women: < 80 cm normal · 80–87 cm to monitor · ≥ 88 cm high

How to improve your BMI

If you are overweight or obese, the recognized levers:

Moderate calorie deficit: 300–500 kcal/day below TDEE. Faster is not better: losing too quickly comes at the expense of lean mass.
Regular physical activity: 150 min/week of moderate activity + 2 strengthening sessions.
Adequate protein intake: 1.6–2.2 g/kg to preserve lean mass during loss.
Sleep and stress: sleep < 6 hours promotes weight gain. Chronic stress too.

What to do with your result

BMI alone is never enough. Combine it with:

The fat mass rate to distinguish between muscle and fat
The TDEE to set a calorie goal
The waist-hip ratio to determine fat distribution

In case of doubt or rapid change, consult your doctor. BMI is a screening tool, not a medical diagnosis.

À consulter aussi

TDEE — dépense totale Énergie quotidienne totale Taux de masse grasse Méthode US Navy BMR — métabolisme de base Énergie au repos Ratio taille / hanches Risque cardio-métabolique

Questions fréquentes

No, not alone. A muscular athlete often has a BMI in the “overweight” zone without any real excess fat. Combine with body fat percentage and waist circumference for an accurate reading.

BMI is mathematical (weight/height²), it does not vary. But the interpretation may differ depending on the scale (WHO, IOTF). Ours follows the WHO adult reference classification.

The healthy range is BMI 18.5–24.9. On 1m70, this gives approximately 53–72 kg. This is a wide range: do not aim for a specific BMI, but the bottom of the normal zone as a benchmark.

The BMI gives a statistical health range, the ideal weight formulas (Lorentz, Creff) give a single value. Both are benchmarks, to be compared with real body composition.