What BMI was designed to do
Body mass index is weight in kilograms divided by height in metres squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the characteristics of populations, not diagnosing individuals — he explicitly described it as a measure for describing groups.
It became a clinical shorthand in the 1970s largely because it is cheap: it needs only a set of scales and a tape measure, and it correlates reasonably well with body fat across large populations. For epidemiology it remains genuinely useful, which is why it persists.
The standard adult categories are: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 and above obese, with obesity subdivided at 35 and 40. These thresholds are conventions that have themselves changed — the US lowered its overweight threshold from 27.8 to 25 in 1998, reclassifying millions of people overnight without anyone's body changing.
Who it misclassifies, and why
BMI measures mass, not composition, and cannot distinguish muscle from fat. Muscular people are routinely classified as overweight or obese despite low body fat — this is systematically true of athletes, and it is the most widely known limitation.
The reverse error matters more clinically and is less discussed. Someone within the healthy range can carry a high proportion of fat and little muscle, a pattern sometimes called normal-weight obesity, and carries elevated metabolic risk that BMI reports as fine. Older adults are particularly affected, since muscle mass declines with age while weight may not.
There are also population differences. Risk of type 2 diabetes and cardiovascular disease rises at lower BMI values in people of South Asian, Chinese and other Asian descent, which is why the UK's NICE guidance and the WHO use lowered thresholds for these groups — overweight from 23 and obesity from 27.5. Applying the standard cut-offs universally understates risk for a large share of the world's population. BMI also does not apply to children and teenagers, who are assessed against age-and-sex percentile charts, nor to pregnant people.
More informative measurements
Waist circumference is a better single indicator of metabolic risk than BMI, because fat stored around the abdomen is more strongly associated with cardiovascular and metabolic disease than fat stored elsewhere. Commonly cited thresholds are above 94 cm for men and 80 cm for women, with higher-risk levels at 102 cm and 88 cm, and lower thresholds again for South and East Asian populations.
Waist-to-height ratio is simpler still and travels better across populations: keeping your waist under half your height is the usual rule, and several analyses have found it predicts risk at least as well as BMI. It needs no lookup table and no adjustment for sex.
The honest summary is that no single number describes health. Blood pressure, blood lipids, blood glucose, fitness, diet, sleep and smoking status all matter, and several are more modifiable than weight. This calculator gives an estimate from two measurements and nothing more — it is not a diagnosis and not medical advice. If you have concerns about your weight or health, speak to a doctor or a registered dietitian, who can interpret these numbers alongside everything else about you.