BMI is probably the most famous — and most criticised — health measurement in existence. Doctors use it, insurance companies reference it, and health apps display it prominently. Yet scientists and health professionals have been arguing about its limitations for decades. So what's the story? Is BMI useful, useless, or something in between? Let's dig in.
What Is BMI?
BMI stands for Body Mass Index. It's calculated by dividing your weight in kilograms by the square of your height in metres: BMI = weight (kg) ÷ height² (m²). If you weigh 70kg and are 1.75m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9. Our BMI calculator does this instantly.
The standard categories are: Underweight (below 18.5), Healthy weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). These ranges are used globally, though some health organisations use slightly different thresholds for different ethnic groups.
Where BMI Came From
BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s. He wasn't trying to measure individual health — he was studying the "average man" in populations for statistical purposes. The measure was adopted as a clinical tool decades later, somewhat outrunning its original intent.
What BMI Gets Wrong
The fundamental problem with BMI is that it measures the relationship between your height and weight — nothing else. It has no way of distinguishing between fat, muscle, bone density, or water. This leads to some absurd results:
- A highly muscular athlete might show as "overweight" or even "obese" because muscle is denser than fat.
- An older person with low muscle mass might show as "healthy" BMI while actually having dangerously high body fat.
- Different ethnicities carry health risk at different BMI levels — South Asian people, for instance, face cardiovascular risk at lower BMI values than European populations.
Researchers have found that BMI misclassifies health status in a significant proportion of people — roughly 30% by some estimates. It's a blunt instrument applied to a nuanced problem.
What Matters More: Body Fat Percentage
Body fat percentage directly measures what BMI is trying to approximate. A healthy body fat range is roughly 10-20% for men and 18-28% for women (ranges vary by age). You can check yours using our body fat calculator, which uses measurements like waist and hip circumference to estimate your fat percentage.
Waist Circumference: An Often-Overlooked Metric
Research consistently shows that where you carry fat matters as much as how much of it you have. Abdominal fat — the kind that accumulates around your organs — is significantly more dangerous than fat stored around your hips or thighs. Waist circumference is a simple, powerful health indicator:
- Men: increased risk above 94cm, high risk above 102cm
- Women: increased risk above 80cm, high risk above 88cm
You don't need any equipment beyond a tape measure. It takes ten seconds. And it predicts cardiovascular risk better than BMI in many studies.
Waist-to-Height Ratio: Even Simpler
An increasingly popular measure is the waist-to-height ratio. The guidance is straightforward: your waist circumference should be no more than half your height. So if you're 180cm tall, aim for a waist below 90cm. This rule holds up well across different heights and body frames.
Should You Still Use BMI?
BMI is a reasonable population-level screening tool, and it's quick and free. For most people who are clearly within a healthy weight range and have an average body composition, it's a useful rough guide. Where it falls short is at the extremes — very muscular people, older adults with low muscle mass, and those from certain ethnic backgrounds.
The sensible approach is to use BMI as one indicator among several, alongside waist circumference, body fat percentage, and basic blood tests. No single metric tells the whole story.
Practical Takeaway
If your BMI lands in the healthy range and your waist is proportionate to your height, you're likely doing well. If BMI says one thing and other indicators suggest another, trust the fuller picture. Health is a collection of signals, not a single number.
Further reading: The NHS has a clear guide to BMI and weight assessment. Visit the NHS BMI guidance page.