What Is BMI and How Is It Calculated?

The formula, a worked example, and what the number actually tells you — and doesn't.

BMI (Body Mass Index) is a screening number that relates your weight to your height. It doesn't measure body fat directly — but it's the fastest rough check available.

The formula

BMI = weight (kg) ÷ height (m)²

Example: 70 kg at 1.75 m → 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 — squarely in the healthy range.

The categories (WHO)

1. Below 18.5: Underweight
2. 18.5 – 24.9: Healthy weight
3. 25 – 29.9: Overweight
4. 30 and above: Obesity

What it doesn't tell you

BMI can't distinguish muscle from fat. Athletes, bodybuilders and very muscular people often score "overweight" while being extremely fit. It's a screening tool, not a diagnosis — if your number concerns you, a doctor can assess properly.