Body mass index is weight in kilograms divided by height in metres squared. That is the whole formula. It was developed in the nineteenth century as a way of describing populations, not diagnosing individuals, and most of the criticism it attracts comes from being asked to do the second job.
What it does well
BMI is cheap, quick, and needs only a set of scales and a tape measure, which is why it remains standard in public health. Across large groups it correlates well enough with body fat to be useful for tracking trends over time and comparing populations. For an individual, it is a reasonable first look, and a change in your own BMI over months does mean something, because your height is not the variable that moved.
What it cannot see
The difference between muscle and fat
The formula uses total weight, so it has no way to tell dense tissue from fat. A muscular athlete and a sedentary person of the same height and weight get the same number, despite very different body composition. This is the limitation people notice first, and it is real.
Where the weight sits
Fat carried around the abdomen is associated with different health risks than fat carried on the hips and thighs, and BMI is blind to the distinction. A waist measurement, or a waist-to-height ratio, captures something BMI structurally cannot.
Age, build and background
Body composition shifts with age at the same weight, frame size varies between people, and the risk thresholds attached to particular BMI values do not apply identically across all populations. The number is the same everywhere; what it implies is not.
Reading it sensibly
Treated as one input among several, BMI is fine. Treated as a verdict, it is being asked for precision it does not have. Two things make it more useful in practice: look at the trend in your own figure over time rather than a single reading, and pair it with a measurement that captures something different, such as waist circumference or a body fat estimate.
The BMI calculator shows the arithmetic, and the body fat calculator uses circumference measurements to estimate composition rather than total weight alone. Neither replaces a conversation with a doctor, who can weigh these numbers against everything else about your health.
The honest summary
BMI is a ratio, not a diagnosis. It is a screening figure that flags where a closer look might be worthwhile, and it is at its worst when treated as a target to hit. Any number that can be moved by dehydration in a morning deserves to be read with that much context.