How To Calculate BMI

Body mass index is weight divided by height squared, and nothing more elaborate than that. For an adult of 70 kilograms and 175 centimetres, the height becomes 1.75 metres, its square is 3.0625, and the index is 70 / 3.0625 = 22.8571428571. The quantity is dimensionless once the units are consistent, which is why the same person scores 22.8548808 when the arithmetic is carried out in pounds and inches instead; that small gap is rounding rather than a different measurement. The formula is genuinely simple. Almost every problem with BMI lies not in computing it but in reading it.

The index was devised by the Belgian statistician Adolphe Quetelet in the 1830s, and for a century it was used to describe populations rather than individuals. Quetelet was studying how human size is distributed across large groups, and he wanted a single scale that removed the effect of height so that people of different stature could be compared fairly. That origin is worth keeping in mind, because a tool built to summarise a crowd behaves quite differently when it is pointed at one person. The name body mass index arrived much later; the underlying ratio has not changed since.

Metric and imperial versions are the same idea in different clothing. The metric form is BMI = kilograms / metres squared. The imperial form is BMI = 703 x pounds / inches squared, where the constant 703 absorbs both the pounds-to-kilograms and the inches-to-metres conversions. A person of 154.323583526 pounds and 68.8976378 inches scores 22.8548808 by the imperial route and 22.8571428571 by the metric one. The two agree to within rounding, and any larger disagreement is a sign that a unit was entered wrongly. The constant is not a definition but a rounded conversion, so the imperial answer will always sit a hair away from the metric one.

The familiar bands are below 18.5 underweight, 18.5 to 25 normal, 25 to 30 overweight and 30 or above obese. They look like physiological facts but they are statistical conventions, chosen because they separate risk reasonably well across large populations. For a 175 cm adult the healthy band corresponds to 56.65625 kg at the bottom and 76.5625 kg at the top, while the obesity line at BMI 30 sits at 91.875 kg. Shifting a cut-off by a single point moves that boundary by several kilograms, which shows how administrative the lines really are. The word normal in the middle band is itself misleading, because it implies that the surrounding bands are abnormal rather than merely statistically less common.

The most cited limitation is that BMI cannot tell muscle from fat. Muscle is denser than fat, so a lean and heavily trained athlete can carry enough total mass to cross the overweight or obese line while holding very little fat. A 100 kg athlete at 175 cm returns 100 / 3.0625 = 32.6530612245, comfortably inside the band labelled obese, yet a sedentary person of identical height and weight receives exactly the same verdict. The formula weighs the whole body and is blind to what that body is made of.

Even when two people genuinely carry the same quantity of fat, BMI ignores where it sits. Fat around the abdomen and around the internal organs is linked to metabolic risk in a way that fat on the hips and thighs is not, and that distinction is invisible to a weight-and-height ratio. Waist circumference, waist-to-height ratio and waist-to-hip ratio capture distribution directly and are often more informative for an individual. Two people can share a BMI of 22.8571428571 and face quite different risks. A tape measure around the waist is therefore a useful companion to the index, especially for anyone whose weight sits near a cut-off.

The cut-offs were derived largely from European and North American populations, and risk does not begin at the same index everywhere. Several groups, including many South and East Asian populations, appear to develop metabolic problems at a lower BMI, and some health bodies therefore apply lower overweight and obesity thresholds to them. Age adds a further shift: at a fixed BMI, body composition changes across a lifetime, so the same number carries slightly different implications at 25 and at 75. In practice this means two people with the same score can be given different advice depending on their background, which is a strong hint that the number alone is not the whole story.

Children and adolescents cannot be judged with adult thresholds at all. Their bodies are still growing, and the normal range of BMI moves substantially with age and sex throughout development. Paediatric assessment therefore uses percentile charts that place a child against others of the same age and sex, rather than fixed numbers. Pregnancy, and unusually tall or short stature, are further cases where the plain adult cut-offs fit poorly and should be set aside. For the same reason, comparing a child's index with an adult's is meaningless, however similar the two numbers happen to look.

Above all, BMI is a screening tool rather than a diagnosis. A single index cannot confirm or exclude any health condition, and clinical assessment uses waist measurement, body composition and blood markers alongside it. What BMI does well is cheap, quick and reproducible: it needs only a scale and a tape measure, it is easy to follow in one person over time, and it remains useful for comparing large groups. Read as a first signal that may prompt a closer look, rather than as a final verdict, it earns its place.