BMI Explained: What It Measures, What It Misses
BMI is a useful screening tool but it isn't the whole story. Here's how it works, when it's misleading, and what to look at instead.
Body Mass Index (BMI) has been the default health screening metric for decades. It's simple, cheap, and correlates with population-level risk. But it also has real limitations you should know before treating your BMI number as gospel.
How BMI is calculated
BMI = weight (kg) / height (m)². The WHO groups adults into underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), and obese (≥30).
Where BMI shines
For large groups, BMI predicts cardiovascular and metabolic risk reasonably well. It's easy to measure — no expensive equipment required — which is why every doctor's office uses it.
Where BMI misleads
BMI can't tell muscle from fat. A muscular athlete may show "obese" BMI without excess body fat. Older adults can lose muscle mass while gaining fat with a stable BMI. And BMI thresholds don't account for how fat is distributed — visceral fat around the abdomen carries more risk than fat elsewhere.
Better measures to pair with BMI
- Waist-to-height ratio (aim under 0.5)
- Body fat percentage via DEXA or bioimpedance
- Simple tape measurements (see the Body Fat Calculator)
Bottom line
Use BMI as one data point, not the whole picture.