What BMI Really Tells You — and What It Doesn't
Body Mass Index is probably the most quoted — and most argued-about — health number in the world. It is calculated from nothing but height and weight, which is precisely its strength and its weakness. Used correctly, BMI is a fast, free screening signal. Used as a verdict on an individual's health, it regularly misleads. Here is how to get value from it without over-reading it. What BMI Really Tells You — and What It Doesn't Body Mass Index is probably the most quoted — and most argued-about — health number in the world. It is calculated from nothing but height and weight, which is precisely its strength and its weakness. Used correctly, BMI is a fast, free screening signal. Used as a verdict on an individual's health, it regularly misleads. Here is how to get value from it without over-reading it. 01 . What the number means BMI divides your weight in kilograms by your height in meters squared. The World Health Organization brackets the result into four screening bands: under 18.5 is classified as underweight, 18.5–24.9 as the healthy range, 25–29.9 as overweight, and 30+ as obese. At the population level these bands correlate clearly with risks for cardiovascular disease, type 2 diabetes and joint problems — which is why doctors and insurers still use them. 02 . Where BMI misleads BMI cannot tell muscle from fat. A 90 kg rugby player and a 90 kg sedentary office worker of the same height get the same score, despite radically different health profiles — muscular athletes routinely screen as 'overweight'. The opposite failure also exists: so-called normal-weight obesity, where a person with a healthy BMI carries little muscle and a high fat percentage, along with the elevated metabolic risk that comes with it. Age and ethnicity shift the picture too. Older adults naturally lose muscle, so the same BMI represents more fat than it would in a younger person. Several health agencies use lower cutoffs for people of South and East Asian descent because metabolic risk rises at lower BMI values in those populations. Children and pregnant women should not use the adult scale at all. 03 . What to check alongside it Waist circumference is the cheapest upgrade: abdominal fat is the metabolically dangerous kind, and a waist above roughly 102 cm (40 in) for men or 88 cm (35 in) for women flags risk even at a normal BMI. Waist-to-height ratio (keep your waist below half your height) works across body types. And nothing replaces the basics a lab can measure: blood pressure, fasting glucose, and lipid panel. Takeaway: Treat BMI as a smoke detector, not a fire report. It is a fast first signal that costs nothing — but decisions about your health deserve at least a tape measure, and ideally a conversation with a professional.