1. What is BMI?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It is widely used as a screening tool to categorise individuals into weight status groups — underweight, normal weight, overweight, and various classes of obesity.
BMI is not a direct measure of body fat or health. Rather, it is a simple ratio that correlates reasonably well with more direct measures of body fat for most people in population-level studies. It was adopted by the World Health Organization (WHO) and national health bodies as a practical, low-cost screening tool.
BMI is a screening tool, not a diagnostic test. A result outside the "normal" range does not mean you are unhealthy — it simply flags that further assessment may be worthwhile.
2. The BMI Formula
The standard formula for BMI is straightforward:
For example, a person who weighs 70 kg and is 1.75 m tall would have a BMI of:
This would place them in the "Normal weight" category according to WHO classifications.
3. BMI Categories (WHO Classification)
The World Health Organization defines the following standard BMI categories for adults aged 20 and over:
| Category | BMI Range | Health Risk |
|---|---|---|
| Severely Underweight | Below 16.0 | High (malnutrition risk) |
| Underweight | 16.0 – 18.4 | Moderate |
| Normal weight | 18.5 – 24.9 | Low (reference range) |
| Overweight | 25.0 – 29.9 | Increased |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very high |
| Obese Class III | 40.0 and above | Extremely high |
These classifications apply to adults of all genders and most ethnic backgrounds, though there is evidence that certain thresholds may not apply equally to all populations — particularly for people of Asian descent, where health risks may occur at lower BMI values.
4. A Brief History of BMI
BMI was developed in the early 19th century by Belgian mathematician and statistician Adolphe Quetelet, who introduced it as the "Quetelet Index" around 1832. Quetelet's goal was not to measure individual health — he was studying the statistical properties of human populations.
The index was largely forgotten until the 1970s, when physiologist Ancel Keys and his colleagues published research comparing it to other methods of estimating body fat. Keys coined the term "Body Mass Index" and recommended it as the best proxy for body fat percentage in population studies.
The WHO officially adopted BMI as an international standard for classifying overweight and obesity in 1995. Since then, it has become the most widely used screening metric in clinical, epidemiological, and public health settings — largely because it requires only two cheap, easy measurements.
BMI was designed as a population statistics tool in the 1830s — nearly 200 years before modern understanding of metabolic health. This context is important when interpreting individual results.
5. How to Interpret Your BMI Result
When you receive a BMI result, it is most useful as a starting point for conversation with a healthcare professional — not as a verdict.
If your BMI is in the "normal" range
This generally suggests your weight is appropriate for your height. However, it does not guarantee good metabolic health, particularly if your lifestyle involves low physical activity or a poor diet. Some people with normal BMI can have high body fat percentage and poor cardiometabolic health.
If your BMI is in the "overweight" range
A BMI between 25 and 29.9 is associated with increased risk for certain health conditions, but risk varies greatly depending on fitness level, body composition, and other factors. Many athletes and physically active people fall into this range due to muscle mass.
If your BMI is in the "obese" range
Higher BMI values are associated with increased risk for type 2 diabetes, cardiovascular disease, certain cancers, and other conditions. However, risk is not uniform and is strongly mediated by lifestyle, fitness, and genetic factors. Speaking with a GP is advisable.
If your BMI is in the "underweight" range
A BMI below 18.5 may indicate insufficient caloric intake or an underlying health issue. This can be associated with nutrient deficiencies, weakened immune function, and bone density loss. Medical evaluation is recommended.
6. Important Limitations
BMI is a useful population-level screening tool, but it has well-documented limitations when applied to individuals. It does not account for muscle mass, fat distribution, bone density, age, sex, or ethnicity. For a detailed discussion, read our article on the limitations of BMI.
Always discuss your BMI result with a qualified healthcare professional who can consider your full health picture.