What Is BMI?
Body Mass Index (BMI) is a numerical value derived from your weight and height. It's used as a screening tool — not a diagnostic measure — to identify adults who may be underweight, in a healthy range, overweight, or obese. The formula is: BMI = weight (kg) ÷ height² (m).
For example, a person who weighs 75kg and is 1.75m tall has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5. This falls within the "normal weight" range according to WHO and Australian health guidelines.
BMI Categories for Australian Adults
The standard BMI categories used in Australia follow WHO guidelines:
- Under 18.5: Underweight — may indicate malnutrition, eating disorders or other conditions
- 18.5–24.9: Normal (healthy) weight — associated with lowest health risk for most people
- 25.0–29.9: Overweight — moderately increased risk of heart disease, type 2 diabetes
- 30.0–34.9: Obese Class I — substantially increased health risk
- 35.0–39.9: Obese Class II — severely increased risk
- 40+: Obese Class III (morbidly obese) — very severely increased risk
Limitations of BMI
BMI is a useful population-level screening tool, but it has important limitations for individuals:
- Muscle mass: Muscular individuals (athletes, bodybuilders) may have a high BMI despite low body fat. A fit 80kg person at 175cm gets a BMI of 26 — technically "overweight" despite being healthier than average.
- Age: Older adults may have a healthy BMI but higher body fat percentage due to muscle loss. BMI doesn't account for this.
- Ethnicity: Australian and international research suggests that Asian populations may face increased health risks at lower BMI levels (25+ rather than 30+ for obesity cut-off).
- Fat distribution: Where you carry fat matters. Abdominal (visceral) fat is more associated with cardiovascular and metabolic disease than subcutaneous fat. Waist circumference is actually a better predictor of metabolic risk than BMI alone.
The Australian Institute of Health and Welfare recommends using BMI alongside waist circumference for a more complete picture of weight-related health risk.
Healthy Waist Circumference Guidelines (Australia)
According to the AIHW and the National Heart Foundation of Australia:
- Men: Waist under 94cm is low risk; 94–101cm is increased risk; over 102cm is high risk
- Women: Waist under 80cm is low risk; 80–87cm is increased risk; over 88cm is high risk
These thresholds are based on European populations — for Asian Australians, the high-risk waist circumference is lower (90cm for men, 80cm for women).
BMI Trends in Australia
Australia's obesity rates have risen steadily. According to the AIHW, roughly two-thirds of Australian adults are now overweight or obese. Rates are higher in regional and rural areas, and higher among lower-income populations. The rate of obese adults (BMI 30+) has more than doubled since the 1980s.
5 FAQs About BMI
BMI is not used the same way for children. For under-18s, BMI is assessed against age-and-sex-specific growth charts — the result is expressed as a BMI-for-age percentile rather than a fixed category. This is because normal BMI values change significantly during childhood and adolescence. Our calculator is designed for adults. Speak to your GP or paediatrician for a child's weight assessment.
Some evidence suggests that for adults over 65, a slightly higher BMI (around 22–27) may be associated with better outcomes than the standard "healthy" range. This is thought to be because some weight provides a buffer during illness. However, the evidence is not definitive. Frailty, muscle loss and body composition are generally more informative than BMI alone for older adults. Discuss with your GP.
Our calculator shows your "weight to healthy range" figure. This tells you how many kilograms above the upper end of the healthy BMI range (24.9) you are. Note that reaching the exact boundary of "healthy" is not the goal — any reduction in weight for those who are overweight tends to improve health markers, even if you don't reach a "healthy" BMI. A 5–10% reduction in body weight is enough to meaningfully improve blood pressure, blood sugar and cholesterol.
Yes — this concept is sometimes called "metabolically healthy obesity." Some individuals with a BMI over 30 have normal blood pressure, blood sugar, cholesterol and insulin sensitivity. However, research suggests this is often a temporary state — many metabolically healthy obese individuals develop metabolic abnormalities over time. BMI is not destiny, but carrying excess weight does increase risk for most people on average.
Your GP is the best first point of contact — they can assess your full health picture, rule out underlying conditions (e.g. hypothyroidism), and refer to dietitians, exercise physiologists or bariatric services as appropriate. The Commonwealth government's Healthy Weight Guide at healthyweight.health.gov.au has resources. For significant obesity (BMI 40+), bariatric surgery may be discussed — Medicare covers certain procedures.
This calculator provides general health information only. It is not a substitute for medical advice. Please consult a qualified healthcare professional for personalised guidance.