Body mass index (BMI) and waist measurement

Together, body mass index (BMI) and waist size can help work out whether your weight is within the healthy range and whether you are at risk of some chronic conditions. Find out what each means and how to use them.

Body mass index

Body mass index (BMI) is an internationally recognised standard to classify the body weight of adults. Although BMI is not a perfect measure, it is the most useful and valid for adults.

BMI is not an appropriate measure for children. If you are worried about your child’s weight, speak to your doctor.

To calculate your BMI, divide your weight in kilograms by the square of your height in metres. Using a BMI calculator makes this easy.

Meaning of BMI ranges

BMI (adults)


Less than 18.5


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 or more

Obese class III

There are some exceptions – the healthy weight BMI range is generally:

  • lower for people of Asian background
  • higher for people of Polynesian background
  • higher for older people
  • higher for elite athletes with higher levels of lean body tissue
  • higher for pregnant women.

As the BMI calculation doesn’t take fat or muscle into account, you should measure your waist as well to get the full picture.

Waist measurement

Measuring your waist can help predict your risk of some chronic conditions.

A higher waist size shows internal fat deposits, which can coat the heart, kidneys, liver and pancreas. This increases the risk of chronic conditions, like type 2 diabetes, heart disease and some cancers.

Men and post-menopausal women are more likely to develop excess fat in the waist area. Fat that is mainly around the hips and buttocks doesn’t carry the same risk.

Waist size showing increased risk of chronic disease


Increased risk

Greatly increased risk


94 cm or more

102 cm or more


80 cm or more

88 cm or more

Generally, the larger your waist, the higher your risk of developing chronic conditions.

These measurements do not apply to children and pregnant women.

Measuring your waist

To measure your waist:

  • take off any bulky clothing, loosen your belt and empty your pockets
  • stand with your feet shoulder-width apart
  • wrap a tape measure around your belly – in line with your belly button, and loose enough to fit one finger inside the tape.
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