Body Mass Index

Body mass index is your weight in kilograms divided by the square of your height in meters. Squaring the height is what puts people of different sizes on a comparable scale rather than just reporting their weight.

The standard categories are below 18.5 underweight, 18.5 to 24.9 normal, 25.0 to 29.9 overweight, and 30.0 and above obese. They are the same for men and women and are not adjusted for age.

It is popular because it needs two numbers everybody already knows, and across a large group it correlates well with the things researchers care about. That is what it was designed for, and at that job it is genuinely good.

What it is not

It is not a measure of body fat, and it cannot tell muscle from anything else. A heavily built athlete and someone carrying the same weight as fat get the same index, which is the single most important thing to know about it. It also says nothing about where weight is carried, and abdominal fat carries risks that the same weight elsewhere does not.

Where you see it in FitnessSyncer

Put your height in your profile and your BMI is worked out from every weight that arrives from a connected scale or from the Notebook, charted on your Dashboard beside the rest of your body composition. Our BMI calculator works one out by hand.