Body Mass Index (BMI): What It is and How to Calculate It

You might know figures from the Australian Bureau of Statistics show approximately two-thirds of Australian adults are overweight or obese, but have you ever wondered how they come up with these figures? Like many organisations and studies concerned with population health, they use BMI to place people into different weight categories. If your weight falls into the overweight or obese category, you can be at higher risk of developing several serious long-term health conditions including cardiovascular disease, type 2 diabetes and some cancers. Therefore, knowing whether you fit into one of these classifications can help you better understand the risk your weight may have for your health and make informed choices about your weight loss options.

What is body mass index (BMI)?

BMI is a calculation that uses your weight and height to determine which weight category you fall into: underweight, healthy weight, overweight or obese. It also provides a rough estimate of how much body fat makes up your total body weight.

The importance of BMI

While BMI is not an exact measure, it’s the most useful population-level measure for categorising weight because it can be used with adults of both sexes and most ages. It is also inexpensive and easy to measure. For this reason, researchers often use BMI when they’re exploring the link between weight and health conditions. Health professionals may also consider your BMI when working with you to manage your health, with certain BMI figures making you a candidate for different medical weight loss options, including medication or bariatric surgery.

How to calculate your BMI

If you know your weight and height, calculating your BMI is relatively simple. You can determine your body mass index by dividing your weight in kilograms by your height in metres squared (m²).

Understanding your BMI 

Knowing the number is one thing, but what does it mean? Your BMI has implications for your disease risk and health management options. Your BMI will place you into one of four classifications. 

What is an underweight BMI?

If your BMI is less than 18.5, it falls within the underweight range.

What is a healthy BMI? 

If your BMI falls between 18.5 to 24.9kg/m², you’re considered to be within the healthy weight range. Sometimes this will be called a normal BMI.

What is an overweight BMI?

If your BMI is between 25.0 to 29.9kg/m², you’re considered to fall into the overweight category.

What is an obese BMI?

If your BMI is over 30kg/m², you’re considered to be obese.

BMI for women and men is measured in the same way and assessed using the same BMI range. However, women tend to have more body fat than men at the same BMI.

How does BMI change with age?

In addition to gender differences, BMI also varies with age. Children and adolescents are continuing to grow and develop, which makes it difficult to define cut-off points between different weight categories. The BMI test is therefore not typically used in children and adolescents.

Body composition and thus BMI tends to change with age. Older adults typically have more body fat and a different distribution of fat on the body. In fact, a 2020 scoping review found some studies showed that people aged 65+ with a higher BMI may be at lower risk of death from all causes compared to those with a healthy BMI. Therefore, BMI may not be the most appropriate measure of health status for older Australians and you should speak with a doctor to gain a better understanding of the relationship between your weight and health. Researchers continue to explore the associations between age, BMI and health.

Is BMI outdated? 

You may have heard discussions about body mass index limitations, including claims it’s an outmoded way of thinking about weight and health. While it continues to be a useful measure for most adults, it’s true that there are exceptions.

Limitations of BMI

The primary limitation of BMI is that it can’t measure the proportion of weight due to fat versus muscle. This can make BMI a less accurate measure in specific groups, including:

• Body builders, weight lifters and some elite athletes – who typically have high amounts of lean muscle tissue, which weighs more than the same volume of fat
• Pregnant women – who are carrying extra fluid as well as their baby
• People living with a physical disability – who may be affected by muscle wasting
• People from certain ethnic groups, such as Aboriginal and Torres Strait Islander peoples and South Asian, Chinese and Japanese population groups
• People affected by eating disorders such as anorexia nervosa
• People with extreme obesity

Other ways of measuring body mass health

If your health professional believes BMI is not the best tool for you, they may recommend other tests. Some of these include:

• ‍Waist circumference – this can be a better indicator of your disease risk, in particular when measured in conjunction with your BMI. Research shows that measuring your waist circumference both adds to and complements the information about your health risk gained from measuring your BMI.
• ‍Skinfold thickness – this involves measuring fat folds at specific places on the body to calculate a body fat percentage. Studies have shown that this technique may produce average errors of 6.6 kg in adults, with researchers suggesting it should be used with caution in people who are older and those who are obese.‍
• Dual-energy x-ray absorptiometry (DEXA) – which uses X-ray technology to measure muscle and fat. Contemporary DEXA technology can measure body composition with a high level of accuracy.‍
• Underwater weighing – which involves comparing your weight on land to your weight in the water to calculate your body fat. When performed correctly, this method can be accurate to within approximately 2-3% of more advanced methods.
• Bioelectrical impedance – which uses a weak electrical current to estimate body composition. Research shows this method may over- or underestimate body fat by approximately 5% and therefore should be used with caution.‍
• Blood tests – to look at things like blood glucose and cholesterol levels

BMI health risks

It’s important to note that discovering your BMI is not an end in itself. Rather, it helps you and your doctor gain a more complete picture of your health. Overweight and obesity are linked with a higher risk of developing numerous health conditions, including:

• Heart disease and stroke
• Type 2 diabetes
• Some cancers
• Kidney disease
• Gallbladder disease
• High blood pressure
• High cholesterol
• Musculoskeletal problems, such as arthritis and joint pain
• Sleep issues
• Sexual health problems
• Difficulties with pregnancy and labour.

The higher the excess weight, the greater your risk of developing these conditions. Excess weight can also make it more difficult for you to manage any existing health conditions.

Can you change your BMI?

Fortunately, it’s possible to change your BMI and reduce your disease risk! Losing as little as 5% of your body weight can make a difference to your health. With help from professional assessment, advice and tailored strategies, most people can successfully lose weight.

If your BMI is over 30 – or higher than 27 if you have one or more health problems – you might want to speak with a medical professional about your options, particularly if you’ve tried to lose weight before and had trouble keeping it off. 

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