How this is worked out
BMI is weight divided by height squared:
BMI = kg ÷ m²
An 80kg person 1.8m tall has a BMI of 80 ÷ 3.24 = 24.7. Imperial measurements are converted to metric first rather than using the 703 multiplier, so there is one formula rather than two.
The healthy weight range shown is the 18.5–24.9 band converted back to a weight at your height, which is more useful than a single target: it tells you the width of the range you are aiming at.
Waist-to-height ratio is simply waist divided by height, and the guide is to keep it under 0.5 — your waist less than half your height. It outperforms BMI as a single predictor of cardiometabolic risk, it holds across adult heights and both sexes, and it needs a tape measure rather than scales.
A worked example
- Weight
- 80 kg
- Height
- 180 cm (1.8 m)
- Height squared
- 3.24
- BMI
- 24.7
- Category
- Healthy weight
- Healthy range at this height
- 60–81 kg
- Waist 80cm ÷ height 180cm
- 0.44 — under 0.5
BMI was never designed to assess a person
Adolphe Quetelet devised the ratio in the 1830s as a way of describing populations. He was a statistician studying the characteristics of groups, and he said explicitly that it should not be used to judge individuals. The medical adoption came more than a century later, largely because it is cheap to calculate and correlates well enough across large samples.
That history is the reason for every caveat on this page. A measure built to describe a thousand people can be right about the thousand and wrong about you — and it is wrong in predictable directions, which at least makes it possible to say when to distrust it.
The most common failure is muscle. Muscle is denser than fat, so a lean, strong person can land in the "overweight" band while carrying very little fat. Most rugby forwards, and a good proportion of anyone who lifts seriously, are classified as obese by BMI.
The square law breaks at the extremes of height
Dividing by height squared assumes that mass scales with the square of height. Bodies are three-dimensional, so it does not — mass scales closer to the cube. Quetelet chose the square because it fitted the population he measured, not because it describes geometry.
The practical effect is that BMI over-reads for very tall people and under-reads for very short ones. Someone 6'5" gets a figure that overstates their adiposity; someone 5'0" gets one that understates it. The error is small in the middle of the range and grows at the edges, which is why the calculator flags it above 193cm and below 152cm.
Alternative indices exist — the Ponderal index and Trefethen's BMI both use exponents closer to 2.5 — but none has enough clinical adoption to be worth reporting alongside a number your doctor will recognise.
Where the fat sits matters more than how much
Visceral fat — around the organs — carries most of the cardiometabolic risk. Subcutaneous fat, on the hips and thighs, carries much less. BMI cannot distinguish them, which is why two people with identical BMIs can have very different risk profiles.
Waist circumference and waist-to-height ratio capture some of that difference, and they need nothing but a tape measure. The guide is simple enough to remember: keep your waist under half your height. It applies across adult heights and both sexes, which no BMI threshold does.
If you measure one thing, measure your waist. If you measure two, measure your waist and track it over time — the direction is more informative than the number.
Where the thresholds do not apply at all
Under 20, adult categories do not apply. Children and adolescents are assessed against age-and-sex percentile charts because normal body composition changes rapidly through growth; using an adult threshold on a teenager produces a number with no clinical meaning.
Over 65, the relationship reverses. A slightly higher BMI is associated with better outcomes in older adults, probably because it reflects preserved muscle and a reserve against illness. The standard thresholds were not drawn with older adults in mind.
And the risk thresholds differ by ancestry. Several health bodies use lower cut-offs for South and East Asian populations — overweight from about 23 and obesity from about 27.5 — because cardiometabolic risk rises at a lower BMI in those groups. The calculator applies this when you select it.
Assumptions and sources
- Formula
- BMI = weight in kilograms ÷ (height in metres)². Imperial input is converted to metric first.
- Categories
- WHO adult classifications: underweight below 18.5, healthy 18.5–24.9, overweight 25.0–29.9, obesity from 30.0. checked 2026-08
- Asian-ancestry thresholds
- Lower cut-offs used by several health bodies — overweight from about 23, obesity from about 27.5. checked 2026-08
- Waist-to-height ratio
- Guide of under 0.5 for adults. Outperforms BMI as a single predictor of cardiometabolic risk in published comparisons.