What Is BMI and Does It Actually Matter?

Body Mass Index (BMI) is one of the most widely cited health metrics in the world. Doctors use it, insurance companies reference it, and public health campaigns are built around it. But in recent years it has attracted significant criticism. So what exactly is BMI, how is it calculated — and should you care about your number?

How BMI Is Calculated

BMI is a simple formula that divides your weight in kilograms by the square of your height in metres:

For example, someone who weighs 75 kg and is 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 24.5.

In imperial units (pounds and inches), the formula is: weight (lb) ÷ height² (in) × 703.

The BMI Categories

These thresholds were established by the World Health Organisation and are used consistently across most clinical settings. Note that some guidelines use adjusted thresholds for people of Asian descent, where health risks can occur at lower BMI values.

Why BMI Is Useful

BMI's main strengths are its simplicity and consistency. It requires no specialist equipment, can be calculated in seconds, and provides a standardised way to track population-level health trends over time. For large-scale epidemiological research, this makes it invaluable.

At the population level, higher BMI does correlate meaningfully with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, and joint problems. For most people without extreme muscle mass, BMI gives a reasonable rough guide to whether body weight is in a healthy range.

The Limitations of BMI

Here is where it gets complicated. BMI has several well-documented shortcomings:

It cannot distinguish fat from muscle

BMI measures weight relative to height — nothing more. A professional rugby player with very low body fat and significant muscle mass may register as "overweight" or even "obese" on the BMI scale. Conversely, a sedentary person with little muscle but high body fat may sit in the "normal" range despite carrying metabolic risk.

It ignores where fat is stored

Visceral fat — the fat stored around the abdominal organs — is far more strongly linked to metabolic disease than fat stored under the skin. Two people with identical BMIs can have very different health risks depending on their fat distribution. Waist circumference is often a more informative measure of this.

It was derived from a limited sample

BMI was developed in the 19th century by Belgian mathematician Adolphe Quetelet based on data from European men. It was never intended as an individual diagnostic tool, yet that is precisely how it is most often used today.

It varies by ethnicity, age and sex

The relationship between BMI and health risk differs across ethnic groups, age ranges and between men and women. The standard thresholds do not fully account for these differences.

What to Use Alongside BMI

Health professionals increasingly recommend combining BMI with other measures for a fuller picture:

• Waist circumference: A waist above 94 cm (men) or 80 cm (women) is associated with increased health risk.
• Waist-to-height ratio: Many researchers consider this a stronger predictor of cardiovascular risk than BMI alone.
• Body fat percentage: Measured by DEXA scan, bioelectrical impedance, or skinfold calipers — gives a direct measure of fat versus lean mass.
• Blood markers: Blood pressure, blood glucose, and cholesterol levels tell you far more about metabolic health than weight alone.

The Bottom Line

BMI is a useful screening tool — a starting point, not a verdict. It is good at identifying broad population trends and flagging potential concerns worth investigating further. But it should not be used as the sole measure of health, particularly at the individual level.

If your BMI falls outside the healthy range, it is worth discussing with your GP rather than drawing firm conclusions. If it is within range but you have other risk factors, do not assume all is well. Use BMI as one data point among several.