BMI Calculator: What is my Body Mass Index score?

Digital Editor · 8 July 2020
What’s a Body Mass Index (BMI) score and how does it work? A leading dietician explains how it could be used to improve health.
BMI Calculator
Image: One Photo (Shutterstock)

What is the Body Mass Index (BMI)?

The Body Mass Index (BMI) is a scale used by health professionals around the world that can help to assess if a person’s weight is within a healthy range, and to help track weight gain. First theorised in the 1800s by statistician and sociologist Adolphe Quetelet, it has been in wide use since the 1970s, when scientists began to notice a correlation between a population’s BMI range score and their risk of developing certain health issues such as cardiovascular disease, Type 2 diabetes, high blood pressure, some types of cancers and elevations in other risk factors. The calculation used is the same for everyone, regardless of factors such as gender, age or body type.

How do I work out my BMI score?

Your BMI score is used to plot where you would sit on the Body Mass Index, which categorises scores into ranges – underweight, normal weight, overweight and obese. The score is worked out via an equation.

First, square your height in metres (height multiplied by height). Then take your weight in kilograms and divide it by that answer (your height squared). So, if a person weighs 45kg and is 150cm tall, the equation would be: 1.5m x 1.5m = 2.25. 45 (kg) divided by 2.25 (height squared) = 20. This means a person with a height of 150cm and a weight of 45kg would have a BMI score of 20. According to the BMI, that person would be in the “healthy weight range”. Medical experts advise that this equation only applies to adults, and should not be used as a guide for children.

What does my BMI score mean?

It’s important to remember that “researchers have found BMI to be a good indicator for the health and lifespan of adults – not necessarily for an individual, but for a group of people who have the same BMI”, according to the Australian Government’s Department of Health. This means that while it can be a useful indicator when thinking about a large group of people, it may not necessarily line up with how healthy you are personally. The calculation used to find BMI is the same for men and for women, for example.

The different categories for BMI are:

  • Less than 18.5 – Underweight
  • 18.5-24.9 – Healthy weight range
  • 25-29.9 – Overweight
  • 30-plus – Obese

Source: Australian Government Department of Health and National Health and Medical Research Council

The impact your BMI score could have on your health, compared to someone else, can also be influenced by many factors. Nutrition Australia spokesperson Aloysa Hourigan told Canstar that BMI scores can be a useful indicator of a person’s risk of developing chronic lifestyle diseases and other health problems. However, she added that the index had its limitations and should be used in conjunction with other health data, such as waist measurements and a professional nutritional analysis, when it comes to making judgements about health.

BMI Calculator
The Australian Institute of Health and Welfare created this graph showing what proportion of Australian men (18 years and over) fall into the various BMI categories. Source: AIHW, Australian Bureau of Statistics

“It is a population health indicator so may not apply to all individuals,” Ms Hourigan said. “For example, if someone has a very high level of muscle mass, they will weigh more than someone of the same height and stature but with a lower muscle mass. Also, there are differences in healthy weight indicators depending on ethnic background. For example, for people with Asian background, it is typically more appropriate to aim for the lower end of the healthy weight range (but not below the range).

BMI Distribution
The Australian Institute of Health and Welfare created this graph showing what proportion of Australian women (18 years and over) fall into the various BMI categories. Source: AIHW, Australian Bureau of Statistics

“The healthy weight range also changes with age. For those over 65 years old, the healthy weight range is considered to be a BMI score of between 22 and 30.”

The Victorian Government’s Better Health information service also states that BMI alone “isn’t always the best measure of health”.

“Differences in BMI between individual adults of the same age and sex are usually due to body fat, however there are many exceptions to this rule, which is why a BMI figure may not always be accurate,” the website states. “Generally speaking, the more body fat you’re carrying, the higher your health risk. However, BMI cannot differentiate between body fat and muscle mass.”

BMI calculations, the website states, will overestimate the amount of body fat for body builders, high-performance athletes and pregnant women, while underestimating it for the elderly, people with a disability and people at risk of muscle wasting. Height also impacts accuracy, as it can overestimate obesity for shorter people (less than 150cm) and overestimate it for tall people (more than 190cm).

Why is my waist circumference important?

The Heart Foundation says it can be useful to combine BMI with waist measurement, to work out if you may need to address weight-related health. This is because it has been shown that carrying extra kilos around the middle of the body can be an indication of internal fat deposits around vital organs, such as the heart and liver. These types of deposits have been shown to increase risks of heart disease and stroke, among other illnesses, the Heart Foundation explains.

The Heart Foundation states that “your health is at risk” if your waist circumference is greater than:

  • 94cm, if you’re male; or
  • 80cm, if you’re female.

To measure your waist, find the top of your hip bone and the bottom of your ribs, breathe out normally, and wrap a tape measure between those two points.

Ms Hourigan said that “when it comes to identifying risk of developing chronic disease”, waist measurement could often be a more telling indicator than BMI.

“As a dietitian, if a client is aiming to lose body fat, I would suggest they consider both BMI and waist circumference – especially the latter – and would also ask them to consider their weight history, and if there was a weight where they felt fit and healthy. This especially applies if a client says they have been overweight all their life,” she told Canstar.

BMI weight
Source: Vladimir Sukhachev (Shutterstock)

How do I get to a healthy weight?

If you decide that you’d like to aim to get your BMI into the “healthy” range, there’s lots of information and help available. A good place to start researching could be the Australian Government’s website, which includes practical information about healthy eating, exercise and lifestyle choices. However, before starting on any health improvement program, it might be a good idea to see a health professional, such as a GP or dietician.

“Like all things, start with the basics,” Ms Hourigan said. Her advice for people wanting to lose weight is:

  • Consider the what, how, when and why for your food choices and eating pattern. Are you eating at regular intervals over the day? Are your meals and snacks mostly nutritious options that include foods across the core food groups?
  • How well do you listen to your hunger cues? Do you stop eating when you feel full? How quickly do you eat? Is most of your food prepared from fresh ingredients with few additives?
  • How much physical activity do you do regularly?
  • Have you got a sedentary job?
  • Is your main drink water?
  • What is your sleep pattern like?
  • How much does life stress impact your food choices?
  • It may also help to self-monitor your food and exercise patterns – there are many apps that can assist with this.

She said if addressing those factors was not making much difference, it could be a good idea to call on the services of an accredited practising dietitian.

Her advice if you wanted to put on weight:

  • Aim to eat three meals and two or three small snacks per day. Use a good proportion of nutrient-dense foods (foods that have a lot of nutrients in a smaller volume, such as nuts and seeds, nut and seed pastes, cheese, dried fruit and healthy fats and oils).
  • Monitor your food intake: are you missing meals? If so, why is this happening and are there some changes you can make to ensure you take time to eat? Is emotional stress impacting your appetite? You might need to talk to your GP if this is the case.
  • Are you eating nourishing foods?

“Putting on weight can be more difficult than losing weight – so again, consider seeking advice from an accredited practising dietitian,” she added.

Body Mass Index
Image: Ross Helen (Shutterstock)

Does Medicare cover weight loss?

If you meet certain “diagnostic criteria”, such as having a BMI of more than 30, you may also be eligible for a GP Management Plan, which can provide Medicare rebates for a number of appointments with certain health professionals, such as dieticians. Visits to dieticians and other allied health professionals may also be covered if your GP writes you a referral. It’s generally best to check if the appointment would be covered by Medicare before you go.

Does private health insurance cover weight loss programs?

If you have private health insurance, it could be a good idea to check if your policy includes cover for lifestyle programs. Many extras policies offer rebates for services such as dieticians, health club memberships and other allied health services designed to boost overall health.

Many health insurers have smartphone apps which offer lists of the services available and how much cover you can claim under your policy. However, it could be a good idea to check with your health provider and insurer what the costs will be before you attend any appointment.

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Cover image: Vladimir Sukhachev (Shutterstock)


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