What Is BMI?
Body Mass Index (BMI) is a simple numerical value calculated from a person's height and weight. It's used by healthcare providers, insurers, and public health bodies worldwide as a quick screening tool for weight categories associated with health risks.
How to Calculate BMI
The formula is straightforward:
- Metric: BMI = weight (kg) ÷ height² (m²)
- Imperial: BMI = (weight (lbs) ÷ height² (inches²)) × 703
Standard BMI Categories (WHO Classification)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
At the population level, these categories do correlate with certain health risks. However, applying them to individuals is where BMI's limitations become significant.
The Real Limitations of BMI
1. It Doesn't Distinguish Fat from Muscle
BMI measures total body mass relative to height — it has no way to differentiate between fat tissue and muscle tissue. A highly muscular athlete can register as "overweight" or even "obese" by BMI despite having very low body fat and excellent metabolic health. Conversely, someone with a "healthy" BMI can carry a high proportion of body fat ("normal weight obesity"), which carries its own health risks.
2. It Ignores Fat Distribution
Where fat is stored matters as much as how much fat you carry. Visceral fat (stored around internal organs in the abdomen) is far more metabolically harmful than subcutaneous fat (stored under the skin). BMI provides no information about fat distribution.
3. It Doesn't Account for Age, Sex, or Ethnicity
Body composition naturally changes with age — older adults tend to have more body fat at the same BMI as younger adults. Women generally carry more body fat than men at equivalent BMI levels. Research also suggests that people of certain ethnic backgrounds face higher health risks at lower BMI thresholds than the standard categories indicate.
4. It's a Screening Tool, Not a Diagnostic One
BMI was designed as a population-level statistical tool in the 19th century — not as a clinical diagnostic for individual health. Using it as a definitive measure of an individual's health or body composition is a misapplication of what it was designed to do.
More Informative Alternatives and Complements
Waist Circumference
A simple tape measure around the narrowest part of the abdomen provides valuable information about central (visceral) fat. General risk thresholds commonly referenced by health organisations:
- Increased risk: Over 94 cm (37 in) for men; over 80 cm (31.5 in) for women
- High risk: Over 102 cm (40 in) for men; over 88 cm (34.5 in) for women
Waist-to-Height Ratio
Dividing your waist circumference by your height accounts for body size. A ratio of 0.5 or below is generally considered healthy for most adults — this is arguably a better single predictor of cardiometabolic risk than BMI.
Body Fat Percentage
Directly measuring the proportion of your body that is fat tissue is more meaningful than BMI. Methods vary in accuracy and accessibility:
- DEXA scan: Gold standard, highly accurate, but requires specialist equipment.
- Hydrostatic weighing: Very accurate, less commonly available.
- Bioelectrical impedance (smart scales): Convenient and accessible, though accuracy varies by device and conditions.
- Skinfold callipers: Requires trained practitioner; reasonably accurate when done correctly.
Blood Markers
Metabolic health is best assessed through blood tests — fasting glucose, HbA1c, cholesterol panel, triglycerides, and blood pressure together paint a far more complete picture of health risk than any external measurement alone.
Should You Still Track Your BMI?
BMI is a useful, free, and instantly available starting point. It's worth knowing your number, especially at the population-screening level. But it should be viewed as one data point among several — not a verdict on your health. Use it alongside waist measurements, fitness levels, energy, blood markers, and how you feel day to day for a genuinely complete picture.
Health is multidimensional. Any single number — including BMI — will always be an oversimplification.