What is body mass index (BMI) and does it really matter?

Published July 2025 | 5 min read
Expert contributors: Associate Professor Nick Fuller, University of Sydney; Dr Georgia Rigas, Royal Australian College of General Practitioners (RACGP)
Words by Trudie McConnochie

Doctors are using body mass index less to assess obesity, as more accurate methods emerge. So, what’s replacing it – and why is BMI falling out of favour?

Body mass index (BMI) has long been used by doctors as a means of assessing health. But in recent years, studies have questioned its reliability in determining chronic disease risk.

A 2024 study published in the International Journal of Environmental Research and Public Health noted “BMI-diagnosed overweight persons are sometimes physically and physiologically fit by other indicators”. Now, a 2025 global commission report, published in The Lancet, has recommended a new approach to diagnosing obesity, using a wider range of measures rather than BMI on its own.

What does this mean for your health? Does BMI still matter? We asked some experts to explain things further.

What is BMI?

BMI is a tool used to estimate your total body fat and the best weight (approximately) for your health. It’s calculated by dividing your weight (in kg) by your height (in m²). For example, if you’re 1.7m tall and weigh 65kg, your BMI would be 22.5kg/m².

The BMI ranges are:

• less than 18.5: underweight
• 18.5 to 24.9: healthy weight
• 25 to 29.9: overweight but not obese
• 30 to 34.9: obese class I
• 35 to 39.9: obese class II
• 40 and above: obese class III.

Originally, BMI was designed as a mathematical tool for categorising weight ranges of population groups in the 1830s, based on white European men. But since being endorsed by the World Health Organization in 1995, BMI has been widely used by healthcare professionals to help measure health in a general sense, with a high BMI having the potential to lead to chronic illnesses like heart disease, type 2 diabetes and some cancers.

Why is BMI controversial?

Critics of BMI say it overemphasises the number on the scales without taking into account how the body’s ‘adipose tissue’ (fat mass) is distributed. Associate Professor Nick Fuller from the University of Sydney says chronic disease risk is linked to body fat, not weight.

“People with the same BMI can have very different disease risk profiles, primarily driven by where fat is distributed in their bodies,” he explains. “This is because not all fat is equal. If you have fat stored around your stomach, your risk of chronic disease is much higher than people who have fat stored around their hips, because this is an indicator of how much visceral fat you have. This type of fat deep inside the belly is dangerous as it prevents the organs in the body from doing their job and working efficiently.”

Another criticism of BMI is that it doesn’t account for muscle mass. For example, it gives a misleadingly high BMI for body builders and elite athletes who have muscular physiques, and a misleadingly low BMI for people who are elderly or have disabilities that might cause muscle wasting through low activity levels. It also overestimates body fat in pregnant women.

BMI doesn’t account for ethnic backgrounds either, underestimating body fat in Asian and Indian populations, and overestimating it in people with Aboriginal, Torres Strait Islander or Polynesian backgrounds, which is why there are different BMI ranges for certain ethnic groups.

What are the consequences of relying only on BMI?

Dr Georgia Rigas is the founder and former Chair of the Obesity Management Specific Interest Group at the Royal Australian College of General Practitioners (RACGP). She says Australian doctors are increasingly aware of the limitations of BMI as a sole indicator of chronic disease risk.

“A variety of individual factors – such as personal and family history, ethnicity and lifestyle factors – and clinical assessment measures need to be taken into consideration to help me as a doctor calculate the chronic disease risk for the individual patient sitting opposite me,” she says.

“Relying too much on BMI can miss important health risks or falsely classify people as unhealthy or healthy based solely on weight. It can lead to inappropriate treatment decisions and overlook the need for a more personalised, holistic health assessment.”

There can also be psychological impacts. A 2023 paper published in the European Journal of Internal Medicine found overemphasis on BMI can have negative psychological effects on people with body image issues.

The RACGP updated its Red Book guidelines in 2022 to recommend GPs use BMI in conjunction with waist circumference or other evidence-based measures to assess chronic disease risk, notes Dr Rigas.

What other ways can we assess body fat?

There are several other ways to assess body fat and whether it’s likely to lead to health problems.

• Waist circumference: This is measured with a tape measure around your middle. A waist circumference of more than 80cm for Caucasian women and more than 94cm for Caucasian men suggests an increased risk of chronic disease (there are different cut-offs for certain ethnic groups, Dr Rigas notes).
• Waist to hip ratio: This is calculated by dividing your waist circumference by your hip circumference (measured with a tape measure around the widest part of your buttocks).
• Blood tests: Tests including blood pressure, total cholesterol and glucose checks can help indicate your risk of developing heart disease and type 2 diabetes.

Eligible members 18 years and older can access a free HCF Heart and Diabetes Health Check, delivered by the Victor Chang Cardiac Research Institute, at selected locations and dates*. This can measure your risk levels of having a cardiovascular event in the future, plus provide an indication of whether you have type 2 diabetes or are pre-diabetic.

Lifestyle factors in obesity risk

Here are a few simple lifestyle choices to help manage your weight.

• Physical activity: Aim for 2.5 to 5 hours of moderate physical activity, or 1.25 to 2.5 hours of vigorous activity, a week. Moderate activity means you can still comfortably talk (think a brisk walk), while vigorous activity means you need to gasp for breath while talking (think playing soccer). If you have osteoarthritis of the knee or hip, HCF’s Knee Osteoarthritis Program supports eligible members^{^} who want to improve their knee osteoarthritis symptoms and quality of life.
• A healthy diet: Eat a diet rich in vegies (the more variety, the better), fruit, whole grains, lean meats, nuts, seeds, legumes, eggs, tofu and dairy foods. Limit foods with saturated fats, added sugar and salt. Eligible members⁺ with a heart condition or diabetes can access The COACH Program^®: free phone-coaching support to help you improve your nutrition and overall health.
• Managing stress: Developing relaxation techniques like mindfulness and effective ways of dealing with stressful situations can help lower the stress response in your body that can impact on weight.
• Plenty of sleep: Inadequate sleep can increase your risk of obesity. The Sleep Health Foundation recommends aiming for between seven and nine hours’ sleep a night for Aussies aged 26 to 64, and 7 to 8 hours for those aged 65 and over. Eligible members^# with HCF hospital or extras cover can access a free 12-month subscription to Sleepfit on us.

Need help with your nutrition?

Eating a balanced diet is one of the key areas to maintaining a healthy weight and overall wellbeing. Want some support getting your nutrition on track? Eligible members can get free access or save on the evidence-based CSIRO Total Wellbeing Diet^**. Created by Australia’s national science agency, it combines a higher protein, low-GI eating plan with exercise and proven weight management tools to help improve habits and create lifelong positive behaviours.