BMI Explained: What It Measures and Its Limitations

Body Mass Index (BMI) is calculated by dividing weight in kilograms by height in meters squared. It's used as a quick screening tool to categorize weight status at a population level.

BMI categories

Standard adult BMI categories are: under 18.5 (underweight), 18.5-24.9 (normal weight), 25-29.9 (overweight), and 30 or above (obese). These thresholds are the same for adult men and women.

What BMI doesn't measure

BMI does not distinguish between fat mass and muscle mass. A muscular athlete can have a high BMI without excess body fat, while someone with a 'normal' BMI can still have a high body fat percentage. It also doesn't account for fat distribution, which affects health risk.

When to use other metrics

For a more complete picture, metrics like waist circumference, waist-to-hip ratio, or body fat percentage provide additional context alongside BMI rather than replacing it entirely.

A brief history of BMI

BMI traces back to a formula devised in the 1830s by Belgian mathematician Adolphe Quetelet, who was studying population averages rather than individual health. It was adopted much later, in the 1970s-80s, as a convenient screening tool for large studies — which explains why it was never designed to capture individual body composition.

A 'normal' BMI doesn't guarantee metabolic health

Someone with a BMI in the healthy range can still carry a high percentage of body fat with relatively little muscle — sometimes informally called 'skinny fat' — and can face similar metabolic risks to someone with a higher BMI. This is another reason BMI works best as a starting screening point, not a final verdict.