What BMI measures — and what it misses
BMI is weight in kilograms divided by height in metres squared. It was designed in the 1830s to describe populations, not to diagnose individuals, and it remains a screening signal rather than a verdict.
It cannot distinguish muscle from fat. A trained athlete frequently lands in the "overweight" band with very low body fat, while someone sedentary can sit in the "normal" band carrying substantial visceral fat. Waist circumference and body-fat percentage add the detail BMI omits.
Where the thresholds come from
The cutoffs at 18.5, 25 and 30 come from WHO analysis of mortality and disease risk in largely European-descent populations. Several health bodies use lower thresholds for people of South and East Asian descent, where metabolic risk rises at a lower BMI — often 23 for overweight and 27.5 for obesity.
BMI is also unreliable for children, pregnant people, and anyone over roughly 65, where height loss and body-composition changes distort the ratio.
Frequently asked questions
Is BMI accurate for muscular people? +
No. Muscle is denser than fat, so athletes routinely register as overweight despite low body fat. Use body-fat percentage instead.
What BMI should I aim for? +
There is no single ideal. The 18.5–24.9 band is associated with the lowest average risk across populations, but individual health depends far more on activity, diet, sleep and blood markers than on a single ratio.