What BMI Can and Cannot Tell You

A 190-year-old screening tool

Body Mass Index — weight in kilograms divided by height in metres squared — was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying population averages, not individual health. That origin explains both its strength and its weakness: BMI is cheap, fast, and works well for describing populations, but it was never designed to diagnose any individual.

Health bodies use it as a first-pass screen. A BMI outside the normal range is a prompt to look closer — at waist measurement, blood pressure, blood sugar, family history — not a verdict in itself.

Why there are two sets of cut-offs

The WHO international categories put overweight at BMI 25 and obesity at 30. But research through the 1990s and 2000s showed that at the same BMI, people of Asian descent tend to carry more body fat and develop type 2 diabetes and cardiovascular disease at lower BMI values. The WHO's Asia-Pacific guidelines therefore set lower action points: overweight from 23 and obesity from 27.5.

The difference is not cosmetic. A person of Asian descent with a BMI of 24 is classified "normal" under the international standard but "overweight" under the standard their own national health bodies — in India, China, Japan, and Singapore — actually use. Our calculator supports both standards and shows which one produced your result.

Where BMI misleads

BMI cannot distinguish muscle from fat. Muscular athletes routinely register as "overweight" while carrying very little body fat. Older adults can register "normal" while carrying too little muscle. BMI is not validated for pregnancy, and for children it must be read against age-and-sex percentiles rather than the adult categories.

BMI also says nothing about where fat is carried, which matters clinically: abdominal fat carries more metabolic risk than fat carried elsewhere. A simple complementary check is waist-to-height ratio — a waist measurement less than half your height is a widely used rule of thumb.

How to use the number well

Track your own BMI over time rather than fixating on a single reading — the trend is more informative than the level. Use the category as a prompt for a conversation with a clinician, not a substitute for one. And if you are of Asian descent, make sure whatever tool you use applies the Asia-Pacific cut-offs; a calculator that silently applies only the international standard can tell you exactly the wrong thing.