What is body mass index (BMI)?

Body mass index (BMI) is a number. It's calculated using your weight and height.

Work out your BMI by dividing your weight by the square of your height (height multiplied by height) as follows:

BMI = Weight in kilos/Height in metres x Height in metres

You can also find your BMI using the healthdirect BMI calculator.

The calculator can give you an idea of any health risks based on your personal results.

Weight classification

Once you know your BMI, you can find your weight classification. There are 4 weight classifications that are used.

For adults, the BMI ranges and classifications are:

1. BMI under 18.5: you are considered to be underweight
2. BMI between 18.5 and 24.9: you are in a healthy weight range
3. BMI between 25.0 and 29.9: you are considered to be overweight
4. BMI equal to 30.0 or above: you are considered to be obese

Your BMI is a guide to tell you if you are a healthy weight for your height. A high BMI can mean your chance of getting some chronic conditions is higher. One of the diseases that is more likely to happen if your BMI is high is diabetes.

What are the limitations of BMI?

BMI is not always accurate for showing if you are a healthy weight. The usefulness of BMI can depend on your body shape, age, sex or ethnic origin.

The BMI calculation can't tell what amount of weight is due to fat or muscle. People who are very athletic can weigh more. This is because the muscle in your body weighs more than fat.

BMI is a less accurate test in these groups of people:

• some high-performance athletes
• body builders or weightlifters
• pregnant women
• people with a physical disability
• people under 18 years

BMI is also less accurate in people from the following ethnic groups:

• Aboriginal and/or Torres Strait Islander peoples
• Pacific Islanders
• Maori people
• South Asian people
• Chinese people
• Japanese people

How can I tell if my child is in a healthy weight range?

There is a specific healthy weight calculator for children and teenagers. You can use it to find out if your child is in a healthy weight range.

It's best for children and teenagers to keep to the weight range for their age and sex. This is to support healthy development.

It is important to remember that the BMI number is a guide only. Always speak with your doctor for advice on your child's health and growth.

Why measure waist circumference?

You have fat in different parts of your body. Some people have extra body fat around their middle.

Measuring your waist circumference can be used as a guide to how much 'visceral fat' you have. This is the type of fat that coats your organs. Visceral fat can be harmful to your health because it can cause inflammation.

If you have too much visceral fat it can increase your risk of some health issues.

These include:

• cardiovascular disease
• metabolic syndrome
• type 2 diabetes
• some cancers

Generally, the larger your waist measurement, the more chance you have of developing health problems.

How do I measure my waist circumference?

Waist circumference is the measure of the length all the way around your body at your waist. It is important to measure your waist accurately.

1. Place the tape measure directly on your skin. You should not wear more than one layer of light clothing.
2. The correct place to measure your waist is halfway between your lowest rib and the top of your hipbone. This is roughly in line with your belly button.
3. Breathe out normally and measure.
4. Make sure the tape is snug, without squeezing your skin.

Waist circumference can be less reliable if you are living with overweight or obesity.

Waist circumference and health risks

These are the waist circumference numbers that can show you have an increased risk of disease:

If you are an adult female:

• your risk is increased if your waist measurement is 80cm or more
• your risk is greatly increased if your waist measurement is 88cm or more

If you are an adult male:

• your risk is increased if your waist measurement is 94cm or more
• your risk is greatly increased if your waist measurement is 102cm or more

What are the limitations of waist circumference?

Waist circumference measurements only apply to adults.

They don't apply to:

• children
• pregnant people
• people with a health issue where their abdomen (tummy) is very enlarged
• people from certain non-European backgrounds

These groups may have a different body shape.

What does overweight or obese mean?

Living with overweight and obesity are health conditions of having a higher body weight than is good for your health.

It usually happens because of eating more food than you need or not being physically active.

Certain medicines and medical conditions can also cause overweight or obesity.

More than 6 in 10 Australian adults are living with overweight or with obesity. That number is rising.

What are the health risks of being overweight or obese?

If you are living with overweight or obesity, it can increase your chance of getting serious health problems, including:

• type 2 diabetes
• high blood pressure
• cardiovascular disease
• sleep apnoea
• osteoarthritis
• some cancers

Many of these diseases can be prevented by having a healthy weight and a healthy lifestyle. This includes eating a well-balanced diet and doing regular physical activity.

Even small amounts of weight loss (as little as 5%) can bring health benefits. These benefits include reducing your risk of getting heart disease or type 2 diabetes.

How can I lose weight?

Probably the most successful method for weight loss is to make a change to your lifestyle. Try to eat food that is healthier and do more physical activity or exercise.

Stress can lead to comfort eating. So mental health support can be important too.

For many people, reaching a 'healthy' weight or BMI is unrealistic. Weight loss of 5% (which means losing 5kg if you currently weigh 100kg) is more achievable. You will still get improvements to your health.

Focus on behaviour change and improved health, not weight loss goals.

You may find that you need a more intensive approach. Such as having a very low energy diet or taking weight loss medicine.

Some people who are living with obesity may need bariatric surgery. Your doctor is more likely to suggest it if you also have other risk factors for serious illness. Surgery might be an option if you are living with obesity and haven't been able to reduce your weight by making lifestyle changes.

How else can I reduce my risk of disease?

Waist circumference and BMI are important indicators of health risk. It is not always possible to control all the reasons why you are living with overweight or obesity.

There are several other ways to improve your health, including:

• quitting smoking
• increasing your physical activity
• improving your diet
• reducing your alcohol intake
• treating other risk factors such as high blood pressure and high cholesterol
• ensuring you get enough sleep

Remember, increased physical activity such as exercise will help lower your chance of disease. A healthier diet will also help. Improving your overall health can have other positive changes in your life such as on your mood and energy levels.

Resources and support

Eat for health gives advice on healthy diets, portion sizes and other tips for losing weight.

LiveLighter offers a range of healthy living tools and calculators to help you and your child make positive lifestyle changes.

The Heart Foundation website gives suggestions for making healthy choices.

You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available to speak with you 24 hours a day, 7 days a week.