Two methods that are commonly used to estimate whether you are a healthy weight or not are body mass index (BMI) and waist circumference.

Waist circumference is considered a good estimate of your body fat, especially your internal fat deposits, and your likelihood of developing weight-related disease.

Health professionals often use BMI and waist circumference together to assess whether someone is overweight or obese and to assess their risk of cardiovascular disease and diabetes.

What is body mass index (BMI)?

BMI uses weight and height to determine whether an adult is within the healthy weight range, underweight, overweight or obese.

It provides an estimate of total body fat and your risk of developing weight-related diseases.

BMI is calculated by dividing weight by the square of height as follows:

BMI = Weight (kg)/Height (m)²

Use the healthdirect BMI calculator to work out your BMI.

The calculator indicates any health risks in relation to your BMI or waist circumference, and offers information based on your personal results.

If you calculate your BMI yourself, it is important to make sure you measure your weight in kilograms and your height in centimetres.

To find your weight classification (if you are an adult), see which of these BMI ranges your weight falls into:

Under 18.5 Underweight
18.5–24.9 Healthy weight range
25.0–29.9 Overweight
30.0–34.9 Obesity I
35.0–39.9 Obesity II
40.0 (and above) Obesity III

Limitations of BMI

BMI is less accurate for assessing healthy weight in some groups of people because it does not distinguish between the proportion of weight due to fat or muscle. BMI is therefore less accurate in certain groups, including:

• certain ethnic groups, such as Pacific Islander populations (including Torres Strait Islander peoples and Maori), Aboriginal peoples, South Asian, Chinese and Japanese population groups
• body builders or weight lifters
• some high-performance athletes
• pregnant women
• the elderly
• people with a physical disability
• people with eating disorders
• people under 18 years
• those with extreme obesity.

Why measure waist circumference?

Carrying excess body fat around your middle is more of a health risk than if weight is on your hips and thighs. Waist circumference is a better estimate of visceral fat, the dangerous internal fat that coats the organs. It is therefore a more accurate predictor of cardiovascular risk, type 2 diabetes in women and metabolic syndrome.

How do I measure my waist circumference?

To find out your level of risk, it is important to measure your waist circumference accurately.

1. Place the tape measure directly on your skin, or over no 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 the skin.

Waist circumference and disease risk

These are the waist circumference thresholds that indicate an increased risk of disease:

If you are a woman:

• your risk is increased at 80 cm or more
• your risk is high at 88 cm or more

If you are a man:

• your risk is increased at 94 cm or more
• your risk is high at 102 cm or more

Limitations of waist circumference

Waist circumference is less accurate in some situations, including pregnancy; where the person has a medical condition involving enlargement of the abdomen; for certain ethnic groups; and for children and young people.

What do overweight and obesity mean?

Being overweight and obesity are conditions of excess weight that normally result from either excess energy (food) intake and/or insufficient physical activity.

Certain medications and medical conditions can also cause weight gain.

What are the health risks of being overweight or obese?

Being overweight or obese can directly contribute to a person developing many serious health problems, including:

• type 2 diabetes
• high blood pressure
• cardiovascular disease
• sleep apnoea
• and osteoarthritis.

Many of these diseases can be prevented by maintaining a healthy weight and following a healthy lifestyle, including a well-balanced diet and regular physical activity.

It is encouraging to know that even small amounts of weight loss bring health benefits, including a lower risk of heart disease and type 2 diabetes.

How can I lose weight?

Probably the most successful way to promote weight loss is to have a lifestyle that combines improved nutrition and increased physical activity.

It should also include psychological support - for many people, achieving a ‘healthy’ weight is an unrealistic expectation. A weight loss of 5% (which means losing 5 kg if you currently weigh 100 kg) is more achievable. It will still result in important health improvements.

Your goals should focus on behaviour change and improved health, rather than weight loss.

Some people may need a more intensive approach, such as a very low energy diet, weight loss medication or bariatric surgery. This is especially likely in those who are obese, have other risk factors, or who have been unsuccessful in reducing weight by changing their lifestyle.

How can I reduce my risk of weight-related disease?

While waist circumference and BMI are important indicators of risk, many other factors also contribute to disease.

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

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

Remember, increased physical activity and improved diet will help reduce your risk of disease and have health benefits that are not directly related to weight loss.