The formula and the categories
BMI divides your weight in kilograms by the square of your height in metres. The imperial version multiplies pounds by 703 and divides by inches squared. The World Health Organization's adult cut-offs — 18.5, 25, 30, 35 and 40 — were chosen because population risk for cardiovascular and metabolic disease rises across those boundaries.
Why BMI is useful
BMI needs only two measurements, costs nothing and correlates reasonably with body fat across large groups. That is why it is the default first screen in so many settings: it flags people who may benefit from a closer look without requiring expensive equipment.
Where BMI misleads
The measure cannot distinguish muscle from fat, or tell where fat sits. Athletes with high muscle mass are commonly classified as overweight or obese despite low body fat. Older adults can sit in the normal range with low muscle and high fat — sometimes called normal-weight obesity. And BMI reads identically whether fat is carried on the hips or the abdomen, despite abdominal fat carrying more risk.
What to use alongside it
Add a waist measurement. A waist-to-hip ratio above 0.90 for men or 0.85 for women signals central fat that BMI misses. Adding a body fat percentage gives a fuller picture still. Read together, BMI, waist and body fat describe size, shape and composition.
The healthy weight range is the useful part
Rather than fixating on a single BMI number, use the range it implies. At any height, BMI 18.5–24.9 spans a wide weight band, and being anywhere inside it is a reasonable target. The top of the band is usually the figure worth watching.
Is BMI outdated?
Critics argue it was designed for populations, not individuals, and that it under-diagnoses metabolic risk in some groups. It remains useful as a cheap first filter, but no clinician relies on it alone. Treat it as one input among several.