BMI (Body Mass Index)

BMI: what is it?

BMI was designed in the nineteenth century to describe populations, and it still does that job well. Across large groups it correlates with health risk, it costs nothing, and it needs two numbers anyone can produce. What it was never designed to do is describe an individual. Its entire input is height and weight, so it has no way of knowing whether an extra ten kilos is muscle, fat or a large frame. Coaches need to know both halves of that: why the number persists everywhere, and exactly where it breaks.

The formula and the standard categories

BMI equals weight in kilograms divided by height in metres squared. The internationally used adult categories are below. They apply to adults, not to children, who are assessed against age and sex percentiles instead.

Where BMI fails an individual client

The best known failure is muscular clients. A well trained rugby player or a lifter with a low body fat estimate routinely lands in the overweight or obese band because muscle is dense and BMI counts every kilo the same. Telling that client they are overweight is not just unhelpful, it is wrong.

The less discussed failure runs the other way. Someone with very little muscle and a high proportion of abdominal fat can sit comfortably inside the normal band while carrying real metabolic risk. BMI is silent about where fat is stored, and abdominal fat is the distribution most strongly associated with risk. It also does not adjust for age-related muscle loss, and the risk thresholds themselves differ across ethnic groups, with lower cut-offs commonly applied for South and East Asian populations.

What to use instead, or alongside

BMI becomes useful again the moment it stops travelling alone. Add a waist circumference and you recover the fat distribution information BMI lacks. Add a body fat estimate and you can immediately tell whether a high BMI is muscle or fat.

How to talk about BMI with a client

Clients arrive having been given a BMI by a doctor, an insurer or an app, and often feeling judged by it. The most useful thing you can do is explain what the number can and cannot see, without dismissing it entirely. It is a screening tool that works on crowds and struggles with individuals.

If a client's BMI sits in a category that concerns you, that is a reason to look at waist circumference and blood pressure and, where appropriate, suggest they see their doctor. It is not something you interpret as a diagnosis on your own.