BMI, with what it does and does not mean

Weight divided by height squared. It is quick, free and the number every guideline is written in, which is why it is here — and why it needs the paragraph after it. Everything is computed in your browser.

What BMI is for

BMI was built to compare populations: at the level of thousands of people, risk of several diseases rises with it, and the WHO adult cut-offs — 25 for overweight, 30 for obesity — mark where that rise becomes clear in the data. That is the job it does well.

What it does not know

For one person it knows nothing about composition or distribution. A muscular person can sit above 25 with little fat; an inactive person can sit below 25 with a lot of it around the organs, which is the fat that matters most for health. It does not know your age, your ethnicity — risk at a given BMI differs between populations, and some guidelines use lower cut-offs for South and East Asian adults — or how you feel and function. The class is a screening flag, not a verdict, and a doctor uses it together with waist circumference, blood pressure and blood tests, not on its own.

The body-fat estimate

The calculator also shows an estimate of body-fat percentage from a 1991 equation using BMI, age and sex. It is a population estimate: for an individual it is easily four or five percentage points off, and it cannot tell a lean 90-kilogram lifter from an inactive 90-kilogram office worker of the same height and age. The app uses the same equation when it has no measurement, only to derive fat-free mass for an energy floor, and replaces it with a DXA or Bod Pod result when one exists. It is here so you can see what the app is working from.

In the app

The app records BMI and the estimated or measured fat-free mass as inputs to its floors, shows the error of the estimate, and never uses BMI alone as a reason to plan or refuse a goal — except that it does not plan towards a goal below BMI 20, which is a conversation with a doctor rather than a calculator.