BMI calculator
The Trefethen index, 1.3 × weight ÷ height^2.5, was proposed in 2013 as a correction for the way plain BMI overstates tall people and understates short ones. It is shown for comparison, not as a replacement — no health guidance uses it.
Bands are the World Health Organization thresholds for adults: under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 or above. They are not adjusted for age, sex or build, and different cut-offs are used for some Asian populations.
Every figure here is a screening number for populations, not a diagnosis for a person. BMI cannot tell muscle from fat, reading high for athletes and low for older people who have lost muscle; the ideal weight formulas were written for drug dosing rather than health advice; and waist-to-hip is a risk indicator, not a verdict. Talk to a doctor before acting on any of them.
Body mass index is weight in kilograms divided by height in metres squared. The WHO treats 18.5 to 24.9 as healthy, 25 to 29.9 as overweight and 30 upwards as obese: someone 170 cm and 70 kg is 24.2. The same panel gives the healthy weight range, four ideal weight formulas, and waist-to-hip ratio against the 0.90 and 0.85 thresholds.
How to work out your BMI
BMI was devised in the 1830s by Adolphe Quetelet as a way of describing populations, and it was never meant to assess an individual. It uses height squared while bodies scale closer to a cube, so it drifts high for tall people and low for short ones. It also cannot separate muscle from fat: a well-trained athlete routinely scores as overweight while being nothing of the sort. The two other measures on this page exist because of that weakness, and neither of them repairs it on its own.
The ideal weight formulas, Devine, Robinson, Miller and Hamwi, were fitted for clinical drug dosing, not for health advice. Their real value is the spread between them: at 175 cm the four land within a few kilograms of each other, while the healthy BMI range at that same height covers nearly twenty. A single figure is the wrong shape for the answer, and four formulas quietly disagreeing make the point better than any warning would.
Waist-to-hip ratio asks a different question again: where fat sits, not how much of it there is. Visceral fat around the organs is metabolically active in a way that fat on the hips and thighs is not, so two people at the same BMI can carry very different risk. This is where the waist measurement earns its place, on its own or against height, and why it has become the more commonly recommended screen.
What people use it for
- Checking a BMI against the WHO adult bands
- Finding the healthy weight range for a given height
- Comparing the Devine, Robinson, Miller and Hamwi ideal weight formulas
- Screening for abdominal fat distribution from waist and hip
- Tracking waist change during a diet
- Adding context to a BMI that reads high for a muscular build
Questions
Between 18.5 and 24.9 for adults, by the WHO definition. The range of weights that produces that at your height is shown under the result.
A range, not a single figure. The healthy BMI span at 175 cm runs from about 56.7 to 76.3 kg, and where you sit inside it depends on build and muscle.
Not as it stands. Children are assessed against age-and-sex percentile charts, because normal body composition changes as they grow and fixed thresholds would misread it.
Muscle is denser than fat. BMI sees only total mass, so a muscular person of the same height weighs more and scores higher without carrying excess fat.
No. Lower cut-offs are used for some Asian populations, overweight at 23 and obese at 27.5, because health risks appear at a lower index.
Devine is the best known: 50 kg for men or 45.5 kg for women, plus 2.3 kg per inch over five feet. Robinson, Miller and Hamwi are shown beside it.
They were fitted to different populations for different clinical purposes. The spread between them is a fair measure of their precision.
There is no single ideal weight. Healthy weight is a band, and build and muscle move you around inside it more than height alone does. If you want a target, take the healthy BMI range, and preferably a goal about how you feel and function.
None of them do. A muscular person will exceed every one of these figures while being perfectly healthy.
Below 0.90 for men and below 0.85 for women, by WHO thresholds.
At the narrowest point, or midway between the lowest rib and the top of the hip bone. Be consistent between measurements.
Apple means fat carried around the middle, pear around the hips and thighs. Apple carries more metabolic risk at the same total weight.
For cardiovascular risk it is often more informative, because abdominal fat matters more than total mass. Keeping your waist under half your height is the usual rule, and it holds across sexes and populations.
You can lose fat, but where your body stores it is largely genetic. Visceral fat does tend to go fairly early in weight loss.
The waist measurement, if you only want one. It tracks the fat that carries the most risk, it needs no formula, and it moves when something changes.