What is BMI?
Body mass index (BMI) is a simple calculation using a person's height and weight. It's widely used as a screening tool to identify whether an adult is underweight, in a healthy weight range, overweight, or obese.
The formula was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is now used globally by health organisations including the World Health Organization and the Australian Institute of Health and Welfare.
BMI categories for Australian adults
The following BMI ranges are used by the Australian Government and major health organisations for adults aged 18 and over.
| BMI Range | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | May indicate nutritional deficiency |
| 18.5 – 24.9 | Healthy weight | Lowest risk — aim to maintain |
| 25.0 – 29.9 | Overweight | Increased risk of some conditions |
| 30.0 and above | Obese | High risk — consider medical advice |
Limitations of BMI
BMI is a population-level screening tool, not a personal health diagnosis. It has several well-documented limitations:
- Doesn't distinguish muscle from fat. Athletes and people with high muscle mass may have a high BMI that doesn't reflect excess body fat.
- Ignores fat distribution. Where you carry fat matters — abdominal (visceral) fat carries higher health risks than fat stored elsewhere.
- Varies by ethnicity. Research suggests that people of Asian background may have higher health risks at lower BMI thresholds.
- Doesn't apply to children. Children use age- and sex-specific percentile charts, not the standard adult categories.
For a more complete picture, use BMI alongside waist circumference, waist-to-height ratio, and regular health checks with your GP.
Healthy weight for Australians
According to the Australian Institute of Health and Welfare, around two-thirds of Australian adults are either overweight or obese. The most effective approach to achieving and maintaining a healthy weight combines balanced nutrition, regular physical activity, and adequate sleep — rather than short-term dieting.