BMI: what it tells you, and what it doesn't

Body Mass Index is a fast screening number, not a diagnosis. Here's how it's calculated and where it breaks down.

The formula

BMI = weight (kg) / height (m)²

In imperial units: BMI = 703 · weight (lb) / height (in)²

What the ranges mean

The WHO classifies adults as underweight (< 18.5), normal (18.5–24.9), overweight (25–29.9), and obese (30+). These cutoffs were derived from population-level mortality data — they describe risk trends, not individuals.

Where BMI misleads

BMI doesn't know the difference between muscle and fat. A lean athlete and a sedentary person of the same height and weight get the same BMI. Body composition, waist-to-hip ratio, and lab markers give a fuller picture.

Try it

Use the BMI calculator — then consider it a starting point, not a verdict.