BMI — What It Measures, What It Doesn't, and What to Do With the Number
Body Mass Index is one of the most used and most criticised health metrics in existence. GPs use it, insurance companies reference it, and public health guidelines are built around it. It is also frequently called out as oversimplified, culturally biased, and a poor indicator of individual health. Both things are true — and understanding why helps you use the number sensibly.
What BMI Actually Measures
BMI is simply weight divided by height squared. For metric measurements:
BMI = weight (kg) ÷ height (m)²
A person who is 175cm tall and weighs 75kg has a BMI of 75 ÷ (1.75)² = 24.5.
The standard categories are: under 18.5 (underweight), 18.5–24.9 (healthy weight), 25–29.9 (overweight), 30 and above (obese). These thresholds were established by the World Health Organisation and are used globally.
Why It's Useful
BMI's strength is simplicity. It requires only two measurements — weight and height — and correlates reasonably well with health outcomes at a population level. Studies consistently show that populations with higher average BMIs have higher rates of type 2 diabetes, cardiovascular disease, and certain cancers. This makes it a useful screening tool in public health research.
Its Well-Known Limitations
It doesn't distinguish muscle from fat. A professional rugby player and a sedentary person can have identical BMIs with very different body compositions. Muscle is denser than fat, so muscular people often register as overweight by BMI.
It doesn't account for fat distribution. Visceral fat — fat stored around the organs — is more dangerous than subcutaneous fat stored under the skin. Two people with identical BMIs can have very different health risks depending on where they carry their weight.
It has different implications across ethnicities. Research has found that people of South and East Asian descent tend to have higher metabolic risk at lower BMI thresholds compared to people of European descent. Some health organisations use adjusted thresholds for these populations.
It ignores age and sex. The relationship between BMI and health risk changes with age, and men and women typically carry fat differently.
What to Use Alongside BMI
Most health professionals consider BMI one data point among several. Useful additional measures include waist circumference (a proxy for visceral fat), waist-to-height ratio, body fat percentage, and basic blood markers like cholesterol and blood glucose. No single number tells the full story of someone's health.
The Practical Takeaway
BMI is a reasonable first-pass indicator, not a diagnosis. If your BMI is in the healthy range and you feel well, there's little reason to be concerned. If it's outside the healthy range, it's worth discussing with a doctor alongside other measurements — not treating the number itself as a verdict.