BMI Calculator

What BMI actually measures

Body Mass Index divides weight in kilograms by height in metres squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way to describe populations, not individuals — he was studying the average build of “the normal man,” not screening anyone's health. The formula only reached medical use over a century later, largely because it is cheap to calculate from two numbers everyone already has.

Where the category cut-offs come from

The World Health Organization's bands — underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, obese 30 and above — come from population studies linking BMI to disease risk across large groups. They describe where risk starts rising on average across a population, not a verdict on any one body. Some national bodies, including several in Asia, use lower cut-offs, because the same BMI carries different average health risk in different populations.

What BMI cannot see

The formula has no way to separate muscle from fat, or to see where fat is carried. A heavily muscled athlete can register as “overweight” despite low body fat, and someone at a “healthy” BMI can carry high visceral fat with elevated health risk. Bone density, frame size and age all shift the relationship between BMI and body composition without changing the calculation at all.

Why children and older adults need different rules

BMI categories are built for adults aged roughly 20 to 65. Children and teenagers are assessed on BMI-for-age percentiles against growth charts, because a healthy BMI changes as a child grows. At the other end, some research associates a slightly higher BMI in older adults with better outcomes, possibly because it reflects muscle and reserve rather than excess fat — one more reason a single number applies less cleanly at the edges of the age range.

What to look at alongside it

Waist circumference and waist-to-height ratio add information BMI cannot: where fat is stored matters for risk independently of how much there is. A waist over roughly half your height is a commonly used flag regardless of BMI. None of this replaces an actual conversation with a doctor, who can weigh BMI against your specific history — it just means the number on this page is a starting point, not a diagnosis.