Understanding BMI: What It Tells You and Where It Falls Short
Body Mass Index is the most widely used screening measure for weight-related health risk, adopted by the WHO and used in clinical practice worldwide. It is genuinely useful — and genuinely limited. Knowing both sides helps you use the number sensibly.
How BMI Is Calculated
BMI is weight in kilograms divided by height in metres squared. A person weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. The standard WHO categories are: under 18.5 underweight, 18.5-24.9 normal, 25-29.9 overweight, and 30 or above obese.
The squared height term makes BMI roughly independent of height for typical adult proportions, which is why a single scale works for people of very different statures.
What BMI Is Good For
At the population level, BMI correlates strongly with body fat and with risks of type 2 diabetes, hypertension, and cardiovascular disease. It is cheap, requires no equipment beyond a scale and a tape measure, and provides a consistent way to track large groups over time — which is exactly what a screening tool should do.
For an individual whose build is roughly average, a BMI drifting from 24 to 29 over five years is a meaningful signal worth acting on, regardless of the measure’s imperfections.
Where BMI Misleads
BMI cannot distinguish muscle from fat. Muscular athletes routinely register as overweight while carrying very little fat. Conversely, older adults can hold a normal BMI while having lost muscle and gained visceral fat — a state sometimes called normal-weight obesity. Research also suggests health risks appear at lower BMI thresholds in South Asian populations, leading some bodies to recommend a lower overweight cutoff of 23 for these groups.
For a fuller picture, pair BMI with waist circumference (risk rises above 90 cm for men and 80 cm for women in Asian guidelines) and, where available, a body composition measurement. And for anything beyond screening, talk to a clinician rather than a calculator.