BMI Explained: What It Does \u2014 and Doesn\'t \u2014 Tell You

What BMI actually measures

BMI is your weight in kilograms divided by your height in metres squared. That is all. It was designed as a fast population-level screening tool, and at that job it is genuinely useful: across large groups, higher BMI correlates with higher risk of type 2 diabetes, heart disease and joint problems. The standard adult categories: under 18.5 underweight, 18.5–24.9 healthy range, 25–29.9 overweight, 30+ obese.

Where BMI misleads

BMI cannot see body composition. Muscular people routinely score “overweight” while carrying little fat; older adults can hold a “healthy” BMI while having lost muscle and gained visceral fat. Risk thresholds also differ by ethnicity — several health bodies use lower cut-offs for people of South Asian descent, for example. And BMI says nothing about where fat sits, even though abdominal fat matters most for health risk.

A useful companion check is waist-to-height ratio: keeping your waist under half your height is a simple rule of thumb that captures abdominal fat better than BMI alone.

How to actually use the number

Treat BMI as a smoke detector, not a diagnosis. If it flags you outside the healthy range, that is a prompt to look further — waist measurement, blood pressure, blood sugar, how your clothes fit, how you feel — ideally with a doctor. Track the trend over months rather than obsessing over a single reading; direction matters more than decimals.

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