Body Mass Index is a single ratio devised in the 1830s by Adolphe Quetelet, a Belgian astronomer and statistician hunting for the numerical shape of the “average man”. He was describing populations, not patients. Close to two centuries later it survives because it asks for nothing but a tape measure and a scale, which means it can be applied to a hundred million people who will never stand on a body-composition scanner.
The quantity it captures is weight relative to the square of height. Squaring the denominator is the whole trick: it makes the index roughly comparable between a 1.55 m and a 1.90 m adult, at the cost of making the number respond more sharply to weight gain in short people than in tall ones. A 5 kg gain moves a short person's index further than a tall person's, and that is arithmetic, not a health signal.
Clinically it functions as triage. Guidelines use it to decide who should be offered further investigation — waist measurement, fasting glucose, lipid panel — rather than to diagnose anything on its own.
The tool holds everything in metric internally. If you choose imperial, the height you type is treated as decimal feet and multiplied by 30.48 to reach centimetres, and the weight is multiplied by 0.45359237 to reach kilograms:
BMI = weight_kg ÷ (height_m × height_m)
Two thresholds then produce the healthy-weight span shown in the results, which is simply the same equation rearranged for weight at the 18.5 and 24.9 boundaries:
low = 18.5 × height_m² high = 24.9 × height_m²
The category bands are the World Health Organization adult cut-offs: below 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, then obesity class I (30–34.9), class II (35–39.9) and class III (40 and above). The result is rounded to one decimal place, which is the finest grain the underlying measurements deserve.
Someone 172 cm tall weighing 68 kg:
Switching the same person to imperial makes the unit trap visible. 150 lb × 0.45359237 = 68.04 kg. A height typed as 5.6 ft becomes 5.6 × 30.48 = 170.69 cm, giving 23.4 — not 23.0, because 5.6 ft is 5 ft 7.2 in, not 5 ft 6 in. If you know your height as feet and inches, convert first: 5 ft 6 in is 5.5 ft.
Both can be. The index is a screening trigger, and a trigger firing means “look closer”, not “diagnosis”. A waist measurement, a blood pressure reading and a fasting glucose test tell a clinician far more than the ratio does.
No. The band is a range precisely because healthy bodies sit throughout it. There is nothing better about 19 than about 22 for its own sake, and driving weight down to reach a lower number is how the index gets misused.
Very little. Drinking a litre of water before weighing moves the figure by roughly 0.3. If two readings a week apart differ by less than half a point, treat them as the same reading and compare weekly averages instead.
No. The calculation runs inside your browser and nothing is transmitted or saved.
This page answers “how much do I weigh for my size”. The Body Fat page estimates composition from circumferences, and the BMR page turns weight, height, age and sex into an energy budget. Used together they describe a body far better than any one of them alone.