Body Mass Index is just weight divided by height squared — in metric, that's kg ÷ m². Take the calculator's own default numbers: 170 cm (1.70 m) and 68 kg. Square the height (1.70 × 1.70 = 2.89) and divide the weight by that (68 ÷ 2.89), and you get a BMI of 23.5 — inside the 18.5–24.9 "healthy weight" range. That's the entire calculation; there's no adjustment for age, sex, frame size, or muscle mass, which is exactly where its limitations come from (see below).
BMI numbers can feel abstract, so here's what the category boundaries actually translate to at a height of 170 cm (5'7"), the calculator's default:
| Boundary | Weight (kg) | Weight (lb) |
|---|---|---|
| Underweight below | 53.5 kg | 118 lb |
| Healthy weight up to | 72.2 kg | 159 lb |
| Overweight up to | 86.7 kg | 191 lb |
At a different height those numbers shift — a taller person's "healthy weight" range covers a higher weight in kg, and a shorter person's covers a lower one, since the formula squares height. That's also why a few kilograms near a boundary can flip someone from one category to the next, which is part of why BMI works better as a broad screening signal than a precise individual measurement.
BMI was developed in the 1830s by the Belgian mathematician Adolphe Quetelet as a way to describe body-weight patterns across a population, not to assess any one individual — and that origin is still its biggest limitation today. Because the formula can't distinguish muscle from fat, it routinely overestimates body fat in muscular, athletic people and can underestimate it in people with low muscle mass, including many older adults. It's also known to relate to health risk somewhat differently across population groups — the World Health Organization's 2004 expert consultation, for example, recommended lower BMI action points for some Asian populations, where health risks linked to excess body fat can appear at lower BMI values than the standard cutoffs suggest. None of this makes the number meaningless, but it's why BMI is treated as a first-pass screening tool rather than a diagnosis — the same reason a healthcare provider looking at your BMI will usually also consider waist measurement, family history, and other factors before drawing any conclusion.
No — BMI doesn't distinguish between muscle and fat, so it can overestimate body fat in very muscular people and underestimate it in others. It's a general screening tool, not a diagnosis.
Under 18.5 is classified as underweight, 18.5–24.9 as healthy weight, 25–29.9 as overweight, and 30+ as the obesity range, per WHO standards.
No — children and teens use age- and sex-specific BMI percentiles rather than the fixed adult categories shown here.