The formula

Body Mass Index is one division:

BMI = weight in kilograms ÷ (height in metres)²

In imperial units the same relationship needs a conversion constant:

BMI = 703 × weight in pounds ÷ (height in inches)²

A worked example

Someone 1.75 m tall weighing 78 kg:

The same person in imperial units — 69 inches and 172 pounds — gives 703 × 172 ÷ 4761 = 25.40. The small gap is rounding in the unit conversion, not a different method.

That result sits just over the line into the overweight band, which illustrates something important: at 76.6 kg the same person is classified normal, and at 76.7 kg overweight. The categories are administrative cut-points on a continuous scale, and nothing about a body changes across them.

The standard categories

Healthy weight ranges by height

Because the categories are ratios, they translate into a weight band for any height. The figures below are the 18.5 and 25.0 boundaries:

Where BMI came from — and what it was built for

This is the part that explains most of its problems. The ratio was devised around 1832 by Adolphe Quetelet, a Belgian astronomer and statistician, as part of his work on the average man. He was describing populations, largely of European men, and looking for the mathematical regularities of a group. He was not proposing a way to assess an individual patient, and no health outcome data existed to validate it against.

The name body mass index was attached much later, by the physiologist Ancel Keys in 1972, who tested several height-weight ratios and found Quetelet's the best available proxy for body fatness in population studies — while stating plainly that it was suited to groups rather than individual diagnosis. In 2023 the American Medical Association adopted a policy formally acknowledging BMI as an imperfect measure that should be used alongside other assessments rather than on its own.

So a number designed for demographic statistics in the nineteenth century became a clinical shorthand in the twentieth, and that migration is the source of the mismatch people notice in their own results.

The exponent is a compromise

Dividing by height squared has a known bias. Mass scales with volume, which grows closer to the cube of a linear dimension, while the divisor grows with the square. The consequence is systematic: BMI reads high for tall people and low for short people of identical build. This is why an alternative using an exponent nearer 2.5 has been proposed — the square was chosen because it happened to fit mid-century European survey data reasonably well, not because it reflects how bodies scale.

The thresholds are not universal

Cardiometabolic risk rises at lower BMI values in several populations, particularly South and East Asian, because for a given BMI the average proportion of body fat and of visceral fat is higher. A WHO expert consultation in 2004 recognised this, and many countries now use lower action points — commonly around 23 for increased risk and 25 for high risk, rather than 25 and 30. If you are using BMI at all, use the thresholds your own health service publishes.

Honest limits

BMI does not measure body fat. It measures nothing but mass against height, and it cannot distinguish between:

• muscle and fat — a trained athlete can score in the obese range with low body fat, and this is a real and common misclassification
• fat around the abdomen and fat on the hips and thighs, though the metabolic risk attached to the two differs substantially
• heavy bone structure and excess weight
• fluid retention and tissue

It is not valid as presented for children and teenagers, who need age-and-sex percentile charts because normal body composition changes throughout growth; during pregnancy; for older adults, where height loss from spinal compression inflates the figure while muscle loss hides fat gain; or for anyone with a limb difference or a condition that alters body composition.

A single better companion measure exists and costs nothing: waist circumference, or waist divided by height. Keeping waist below half your height is a widely used rule of thumb, and it captures central adiposity — the part that carries most of the risk — which BMI is blind to.

This is general information, not medical advice. BMI is a screening ratio, not a diagnosis, and no number here can tell you whether you are healthy. Blood pressure, blood lipids, blood glucose, fitness, sleep and family history all matter, and interpreting them together is a job for a doctor who can examine you.

Questions people actually ask

My BMI says overweight but I lift weights. Is it wrong?

For you, effectively yes. Muscle is denser than fat, so a muscular build inflates BMI without the associated risk. Waist measurement and body-fat estimation are far more informative in that case.

Is BMI useless then?

No — it is useful for what it was built for. Across large groups it tracks average risk well and it is cheap and reproducible, which is why epidemiology still uses it. The error is treating a population statistic as a verdict on one person.

What should I track instead?

Waist-to-height ratio is the most informative measure you can take with a tape measure. Beyond that, trends in your own numbers over months are more meaningful than any single reading against a threshold.

Can I use this for my child?

Not directly. Compute it if you like, but interpretation requires a paediatric percentile chart for age and sex, and that reading belongs with a paediatrician.

Is my height and weight stored?

No. The calculation happens in your browser and the figures you enter are never transmitted or saved.