BMI: what it is, how it's calculated, and who it doesn't fit
A checkup report reads “BMI 27.4 — overweight,” and a person who swims three times a week and never eats after six is baffled. Meanwhile a bodybuilder with a ripped six-pack opens an online chart and finds themselves in the “obese” column. Both reactions are honest: the problem is that body mass index was never designed to judge an individual body. Let's look at what it actually measures, where the normal ranges lie, and when the number can't be trusted.
How BMI is calculated
BMI (body mass index) is weight in kilograms divided by the square of height in meters. At 70 kg and 1.75 m, the math looks like this: 70 ÷ (1.75 × 1.75) = 22.9. There's no need to compute it by hand: enter two numbers in the BMI calculator and you'll instantly see the value and its category on the WHO scale.
A curious detail: the formula is almost two centuries old. It came from Adolphe Quetelet, a Belgian statistician who in the 1830s was looking for a way to describe the “average man” for population censuses. The instrument was built to assess thousands of people at once, not to deliver a personal diagnosis — and that's the key to understanding both its strengths and its weaknesses.
Categories on the WHO scale
- below 18.5 — underweight;
- 18.5–24.9 — normal; the widest band on the scale, where most healthy adults fall;
- 25.0–29.9 — overweight;
- 30.0–34.9 — obesity class I;
- 35.0–39.9 — obesity class II;
- 40 and above — obesity class III.
The cutoffs aren't arbitrary: they're tied to health statistics. From 25 onward, the average person's risk of hypertension and type 2 diabetes starts to climb, and past 30 it accelerates noticeably. So “BMI in the green zone” is a good — if not absolute — sign that your weight isn't a threat. One more subtlety: the formula scales poorly at extremes of height — for the very tall it slightly overstates “fatness,” for the very short it understates it. Within the ordinary range, roughly 150–195 cm, the distortion is negligible.
When BMI lies
The formula knows only two numbers — height and weight. Everything in between — muscle, fat, bone, water — is invisible to it. Hence the situations where the index fails spectacularly:
- athletes and strength trainers: muscle weighs more than fat, so an athlete with low body fat can easily score “obese” despite a perfectly healthy body;
- older adults: with age, muscle is lost while fat remains — a moderate index can hide little muscle and excess visceral fat; some researchers use an upper cutoff of 27–28, not 24.9, for people over 65;
- pregnancy: weight gain is normal here, and the adult scale simply isn't applied;
- children and teens: they get age- and sex-specific percentile charts, not “adult” ranges;
- different populations: for people in Asia, the WHO recommends lower thresholds — overweight starts at 23 there.
A fairer picture comes from complements to the index: waist circumference (over 94 cm for men and 80 cm for women is a reason to check visceral fat), a bioimpedance body-composition analysis, and ordinary lab work: blood pressure, glucose, lipid panel. BMI answers the question “is there a reason to take a closer look,” not the question “am I healthy.”
Why BMI caught on is clear too: it needs nothing but a scale and a stadiometer — no equipment, no lab, no money. A cheap, fast metric is perfect for statistics, screenings, and a first appointment; the trick is not letting it become the only verdict.
As a screening tool, BMI works great: one number in ten seconds shows whether weight is even worth discussing. As a diagnosis, it doesn't. Calculate your value in the BMI calculator, check it against the scale, and make the call on the whole picture: waist, labs, how you feel. And remember both ends of the scale — significant underweight is just as much a reason to see a doctor as obesity.