BMI Calculator

How this is calculated

BMI = weight (kg) ÷ height (m)²

Worked example: 70 kg at 175 cm → 70 ÷ 1.75² = 22.9, inside the WHO normal-weight band. The "normal-weight range" line in the result inverts the formula for your height (18.5–24.9 × height²), which is a more useful target than the abstract score.

WHO adult categories

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 and above	Obesity

This is a screening number, not medical advice — BMI can misread muscular and elderly bodies, and it does not apply in pregnancy or childhood. Talk to a health professional about what your number means for you.

Why BMI persists despite its flaws

BMI was devised in the 1830s by Adolphe Quetelet, a Belgian statistician who was studying populations, not patients. He wanted a way to describe the average build of a nation from height and weight records, which were the only body measurements collected at scale. That origin explains both its usefulness and its limits: it is superb for comparing large groups cheaply, and structurally incapable of describing an individual body.

It survived because nothing cheaper replaced it. A BMI needs a tape measure and a scale; the measurements that actually distinguish healthy from unhealthy bodies — visceral fat, muscle mass, cardiorespiratory fitness — need equipment, time or a laboratory. For a clinician deciding who to look at more closely, a free number that is right about populations is worth having. For you, deciding what to do about your own body, it is a starting question rather than an answer.

Where BMI reads a body wrongly

The formula divides weight by height squared and knows nothing else. Muscle is denser than fat, so trained athletes routinely land in the "overweight" band while carrying very little fat — a rugby forward or a competitive swimmer can register 28 and be in better metabolic health than someone at 23. The error runs the other way too: an older adult who has lost muscle can hold a "normal" BMI while carrying enough visceral fat to be at real risk, a pattern common enough to have its own name in the literature.

Height distorts it in a subtler way. Dividing by height squared, rather than cubed, was a fit to nineteenth-century data rather than a law of geometry, and it means very tall people are pushed slightly upward and very short people slightly downward relative to their actual body composition. And because the formula uses only total mass, it cannot see where that mass sits — yet fat around the abdomen carries far more metabolic risk than the same weight on hips and thighs.

What to measure alongside it

Waist circumference is the single most valuable addition, and it needs the same tape measure. Measured at the midpoint between the lowest rib and the top of the hip bone, it estimates abdominal fat directly. Commonly cited thresholds for increased risk are about 94 cm for men and 80 cm for women, rising to 102 cm and 88 cm for substantially increased risk, with lower thresholds recommended for people of South Asian, Chinese and Japanese descent. A simpler version is waist-to-height ratio: keeping your waist under half your height is a rule of thumb that performs surprisingly well and requires no table at all.

Taken together, a BMI and a waist measurement disagree usefully. A high BMI with a modest waist usually means muscle. A normal BMI with a large waist is the combination worth taking to a doctor, and it is precisely the one BMI alone would have told you not to worry about.

Who the standard categories do not fit

The WHO bands above are for non-pregnant adults. Children and teenagers are assessed on age-and-sex percentile charts instead, because a growing body's healthy ratio changes year by year — an adult BMI applied to a twelve-year-old is meaningless. In pregnancy, weight gain is expected and BMI is used only as a pre-pregnancy baseline. Several health authorities in Asia use lower thresholds, with overweight beginning around 23 rather than 25, because metabolic risk appears at a lower BMI in some populations.

If your number sits near a boundary, treat the boundary as arbitrary, because it is. There is no biological event at 25.0. The bands are administrative lines drawn across a continuous distribution of risk, and moving from 24.9 to 25.1 changes nothing about your body.

Using the target range sensibly

The result panel inverts the formula to show the weight range that would place you in the normal band at your height. That is more actionable than the score, but it is still a range rather than a target — the difference between the bottom and top of it is often fifteen kilograms or more, and every weight inside it is, by this measure, unremarkable. If you are trying to change your weight, the rate matters more than the destination: roughly 0.5–1% of body weight per week is the range that most reliably preserves muscle, and faster loss usually costs you the lean tissue that was helping.