BMI: what it is and how to calculate
Updated on June 18, 2026 · by Rafael Rossi
BMI (Body Mass Index) is the quickest way to check whether your weight sits in a healthy range for your height. It's used worldwide because it turns two simple measurements into one number.
The formula
BMI = weight (kg) ÷ height² (m)
Step-by-step example
A person of 70 kg and 1.75 m:
- height²: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9 → normal weight
WHO ranges
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class I |
| 35 – 39.9 | Obesity class II |
| 40+ | Obesity class III |
The limitations (important)
BMI doesn't separate muscle from fat. A muscular athlete may have a high BMI without excess fat; an older person with little muscle may have a "normal" BMI yet excess fat. Waist circumference and body-fat percentage complement the picture.
FAQ
Does it work for children? Not the same way — children and teens use age-based growth percentiles.
Does a high BMI mean I'm ill? Not necessarily. It's a flag to investigate, not a diagnosis. Talk to a health professional.