What BMI actually measures
Body mass index is a simple ratio of weight to height, originally developed in the 1830s as a population-level statistic rather than an individual health diagnosis. It's still widely used today as a quick screening tool because it's cheap and easy to calculate, not because it's the most accurate measure of an individual's health.
The formula
BMI = weight (kg) ÷ height (m)²
Standard categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Why it's a starting point, not a verdict
BMI doesn't distinguish between muscle and fat, doesn't account for where fat is distributed, and uses the same thresholds regardless of age, sex, or ethnicity — all of which affect what a "healthy" weight actually looks like for a given person. It's a reasonable first signal, particularly at a population level, but a single number outside the "normal" range isn't a diagnosis on its own. Athletes with high muscle mass, older adults, and pregnant people are common cases where BMI reads misleadingly. If your number is a concern, a doctor can weigh it against your actual body composition and health history.