BMI, honestly
"My BMI is 27. Am I fat?" The index cannot answer that question for one person, and knowing why is more useful than the number.
What it measures
Weight in kilograms divided by height in metres squared. That is all. It was designed as a way to compare weight across people of different heights in populations, and at that scale it works: the risk of several diseases rises with BMI across thousands of people, and the WHO's adult cut-offs — 25 for overweight, 30 for obesity — mark where the rise becomes clear in the data. Health systems use it because it is free, instant, reproducible and good enough for screening a population.
What it does not measure
Composition and distribution — which is to say, the two things that matter for an individual. Muscle weighs more than fat per litre, so a muscular person reads "overweight" with little fat; an inactive person can read "normal" while carrying a lot of fat around the organs, which is the fat most tied to risk. The index does not know your age (older adults tend to have more fat at the same BMI), your sex, or your ancestry — risk at a given BMI differs between populations, and some guidelines use lower thresholds for people of South and East Asian descent. It is a screening flag, not a diagnosis.
The newer framework
European clinical guidance has moved on from BMI alone. A 2024 framework for the diagnosis and staging of obesity in adults asks clinicians to combine BMI with a measure of fat distribution — waist circumference or waist-to-height ratio — and with the actual presence of complications, before calling anything a disease or deciding on treatment. In other words: the number opens the conversation, and the waist, blood pressure, blood tests and how you function close it. That is what a doctor does with a BMI of 27, and it is what you should expect one to do.
What to do with your number
Use it as one input. If it is above 25, measure your waist and look at the trend of both over months rather than at either on its own. If it is above 30, the framework above is the conversation to have with a doctor — not because a number is a verdict, but because the complications it flags are worth checking. And if you train seriously and it reads high, the reading is probably about muscle; the body-fat page is the next step.
In the app
The app records BMI as an input to its estimates and its floors, never as a judgement, and does not plan towards a goal below BMI 20 — a limit that protects, not a target that is recommended. The BMI calculator shows the class, the healthy-range weights for your height, and the population body-fat estimate the app uses when it has no measurement.