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We offer a BMI calculator on this site, and it comes with a disclaimer. This article is the longer version of that disclaimer.
Where BMI came from
The formula was devised in the 1830s by a Belgian statistician studying the characteristics of populations. It was never designed to assess an individual, and its inventor said as much. It survives because it is cheap: two measurements anyone can take, and a single number.
What it's genuinely useful for
Across large groups, BMI correlates reasonably well with body fat, and group averages track health outcomes usefully. For public-health research and population screening, it earns its place.
It's also a serviceable trend line for one person over time. If your height is constant, a moving BMI is really just a moving weight — and direction of travel can be informative.
Where it breaks down
It cannot distinguish muscle from fat. Muscle is denser. Well-trained people routinely land in the "overweight" band while carrying very little fat.
It says nothing about distribution. Two people with identical BMI can have very different visceral fat, which is the fat that matters most metabolically. Waist circumference is a better single number here.
The thresholds are not universal. Risk associated with a given BMI differs meaningfully across ancestries. Several health bodies use lower cut-offs for South Asian populations, because metabolic risk appears at a lower BMI.
It's misleading at the edges. Pregnancy, older age with reduced muscle mass, children still growing, and people with limb differences all break the assumptions built into the formula.
What to look at instead
No single number replaces it, which is precisely the point. In a consultation I'd typically want:
- Waist circumference, or waist-to-height ratio
- Blood markers — glucose, lipids, and where relevant thyroid function
- What your weight has actually been doing over the past few years
- Energy, sleep, digestion, and how you feel day to day
So should you use the calculator?
Use it as one data point among several, or as a rough starting position before a proper assessment. Don't use it as a verdict on your health, and don't let a number computed from a nineteenth-century population formula override what your labs and your body are telling you.
A measurement is only as good as the question it's answering. BMI answers "where does this person sit in a population distribution?" — which is rarely the question anyone actually cares about.