BMI (Body Mass Index)

BMI (Body Mass Index) is a quick screening number that relates your weight to your height, used worldwide by doctors, insurers and fitness trackers as a first check of whether a person's weight sits in a healthy range. It is calculated from just two measurements:

BMI = weight (kg) ÷ height (m)²

Because it needs only height and weight, BMI is a fast, free and reproducible way to flag weight categories — which is why it has stuck around despite its limits.

How it works

You square your height in metres, then divide your weight in kilograms by that figure. For example, a person who is 1.70 m tall and weighs 70 kg has a BMI of 70 ÷ (1.70)² ≈ 24.2.

The World Health Organization's adult categories are:

• Below 18.5 — underweight
• 18.5 to 24.9 — normal weight
• 25 to 29.9 — overweight
• 30 and above — obese

The same formula is used across sexes, though healthy ranges differ slightly with age and muscle mass, and BMI is not designed for children without age-specific charts.

What it can and can't tell you

The biggest limitation is that BMI treats all weight the same. It cannot distinguish muscle from fat. A muscular athlete can register as "overweight" because muscle is dense, while a person with low muscle mass and high fat can show a "normal" BMI and be unaware of the health risk. Waist circumference and body-fat measures are often better companions for a fuller picture.

Treat BMI as a screening tool, not a diagnosis. It points your doctor toward who may need a closer look rather than deciding your overall health on its own. See where you fall with the BMI Calculator.

A note on its origins

The index is often credited to the Belgian mathematician and statistician Adolphe Quetelet, who in the 1830s devised the measure while studying the physical characteristics of the "average man" across populations. It was formally named the body mass index much later — in 1972 — by physiologist Ancel Keys, who validated it as a useful population-level indicator in a study that compared several obesity measures. That long history helps explain why such a simple formula still anchors so much health screening today, even as researchers continue to debate its precision for individuals.