What Is BMI and How Is It Calculated?
Body Mass Index (BMI) is a numerical value derived from an individual's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been adopted worldwide as a simple, inexpensive screening tool for weight-related health risk.
The formula is straightforward:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = (weight (lbs) ÷ height (inches)²) × 703
For example, if you weigh 80 kg and are 1.78 m tall: BMI = 80 ÷ (1.78 × 1.78) = 80 ÷ 3.1684 ≈ 25.2 (just into the overweight range).
BMI Categories for Adults (WHO Standards)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk of nutritional deficiencies, osteoporosis |
| 18.5 – 24.9 | Healthy weight | Lowest risk for weight-related health problems |
| 25.0 – 29.9 | Overweight | Moderate increased risk of type 2 diabetes, heart disease |
| 30.0 and above | Obese | High increased risk of serious health conditions |
Limitations of BMI
BMI is a useful population-level screening metric, but it has well-documented limitations for individual health assessment:
- Does not distinguish fat from muscle: Highly muscular athletes (such as rugby players or bodybuilders) may have a BMI in the overweight or obese range despite having low body fat.
- Does not measure fat distribution: Abdominal fat (central or visceral obesity) carries higher health risks than fat stored elsewhere. BMI cannot capture this. Waist circumference is a better proxy for metabolic risk.
- Ethnicity considerations: Research shows that people of South Asian, East Asian, and Aboriginal and Torres Strait Islander descent face higher cardiometabolic risk at lower BMI values. Some guidelines recommend a lower overweight threshold (BMI 23) for Asian populations.
- Age and sex differences: Older adults may have more body fat at the same BMI as younger adults, and women typically have more body fat than men at the same BMI.
For a comprehensive health assessment, consult a GP or healthcare professional who can consider your full clinical picture, including blood pressure, blood glucose, cholesterol, waist circumference, and lifestyle factors.
Ideal Weight by Height — Reference Table
The following table shows the healthy weight range (BMI 18.5–24.9) for common heights:
| Height (cm) | Healthy Weight Range (kg) | Height (ft/in) | Healthy Weight Range (lbs) |
|---|---|---|---|
| 155 cm | 44.4 – 59.9 kg | 5'1" | 98 – 132 lbs |
| 160 cm | 47.4 – 63.7 kg | 5'3" | 105 – 141 lbs |
| 165 cm | 50.3 – 67.8 kg | 5'5" | 111 – 149 lbs |
| 170 cm | 53.5 – 72.0 kg | 5'7" | 118 – 159 lbs |
| 175 cm | 56.7 – 76.3 kg | 5'9" | 125 – 168 lbs |
| 180 cm | 59.9 – 80.8 kg | 5'11" | 132 – 178 lbs |
| 185 cm | 63.3 – 85.3 kg | 6'1" | 140 – 188 lbs |
| 190 cm | 66.8 – 89.9 kg | 6'3" | 147 – 198 lbs |
Healthy Weight Tips
If your BMI is outside the healthy range, small consistent changes are more effective than drastic interventions. The Australian Government's health guidelines recommend:
- 150–300 minutes of moderate-intensity physical activity per week
- Eating a diet based on vegetables, fruits, wholegrains, and lean proteins
- Limiting processed foods, added sugars, and alcohol
- Staying adequately hydrated
- Prioritising sleep (7–9 hours per night for most adults)
If you have concerns about your weight or general health, the first step is a conversation with your GP. They can arrange relevant health checks and refer you to a dietitian or exercise physiologist if needed.