What BMI Can Tell You, and What It Honestly Can't

Where the formula comes from

BMI is weight in kilograms divided by height in meters squared. A person of 1.75 m and 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. The formula was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who wanted to describe the AVERAGE build of a population, not to diagnose anyone.

The familiar categories (under 18.5 underweight, 18.5-25 normal, 25-30 overweight, over 30 obese) were standardized by the WHO in the 1990s. They are population-level statistical cutoffs; the boundaries are round numbers chosen for convenience, not biological thresholds where risk suddenly jumps.

What it is genuinely good for

Across large groups, BMI correlates well with body fat and with the risks that follow it: type 2 diabetes, cardiovascular disease, joint problems. That is why doctors, insurers and researchers keep using it; it costs nothing, needs only a scale and a tape measure, and works at scale.

For an individual, BMI is most informative as a TREND. Your muscle mass, bone density and frame do not change month to month, so if your BMI drifts from 24 to 28 over two years, that drift is real information about you, even if the absolute number is debatable.

Where it misleads

Muscle weighs more than fat. A muscular athlete can be 'overweight' by BMI while carrying less fat than average; many rugby players and sprinters land above 25. Conversely, an inactive person can hold a 'normal' BMI while carrying too much fat and too little muscle, a pattern sometimes called normal-weight obesity.

It says nothing about WHERE fat sits, and location matters: fat around the organs (visceral fat, the classic belly) carries far more risk than fat on hips and thighs. Waist circumference or waist-to-height ratio captures this; BMI cannot.

The same cutoffs do not fit all bodies equally. At the same BMI, health risks tend to appear earlier in people of South and East Asian descent (several countries use lower thresholds accordingly), and the standard ranges also fit children, the elderly and pregnant women poorly; children need age-specific percentile charts.

How to actually use the number

Treat BMI as a smoke detector, not a diagnosis: cheap, always on, occasionally wrong in both directions. A reading far from the normal range is a reason to look closer, not a verdict.

Pair it with one more measurement to cover its blind spot: keeping your waist below half your height is a simple, evidence-backed complement that catches the visceral-fat cases BMI misses.

And the honest disclaimer that any BMI page owes you: this is general information, not medical advice. Bodies differ, and a conversation about your weight and health belongs with a doctor who can see the whole picture, not with a formula from 1832.