What is body mass index (BMI)?

BMI uses your height and weight to work out your body size. But it has limitations – so when should it be used?

In this article

BMI and heart disease

BMI is one of several tools a health professional will use to work out your risk of heart attack and stroke. BMI alone is not a good indicator of your heart health or overall health.

It must be used with other heart disease risk factors such as your blood pressure and cholesterol, your ethnicity, your smoking status, and your personal and family health history.

Find out more about your risk of heart disease by using My Heart Check .

A waist measurement (the measurement around your tummy) is a better indicator of where fat sits on your body. When BMI is measured alongside your waist measurement, it better reflects your heart disease risk1,2.

How accurate is BMI?

BMI is a guide for body size and isn’t accurate for everyone. The following need to be considered alongside your BMI measurement:

AgeAs you get older, having a higher BMI can be helpful to protect against malnutrition or from having a fall.
GenderMen and women have different levels of fat. The fat is often in different places on the body too.
EthnicityBMI is less accurate for certain ethnic groups.
People of Asian descent can have more body fat on a smaller frame so for this group, a healthy BMI range would be lower.
People who are Māori or Pasifika often have a higher bone density and more muscle mass. In this group, a healthy BMI range would be higher.
Body compositionBMI is unable to measure:
  • the amount of fat and muscle in your body
  • whether you have excess body fat
  • where fat is located on your body.

BMI does not recognise that some people in the healthy BMI weight range can have higher levels of body fat that you can’t see. This can be more of a risk than someone with a BMI above the healthy range that has more muscle.

BMI is less accurate in very muscular or athletic adults because muscle weighs more than fat. People with a lot of muscle can be classed as ‘above the healthy weight’ range even if they’re lean and fit.

When is BMI helpful?

BMI alone shouldn't be used to determine your risk of heart disease.

However, BMI can be useful to help your health professional see changes to your weight over time. If your health professional sees your BMI increase, and you’re not trying to build muscle, this may show weight gain from fat that might be a risk to your heart health.

Who should not use BMI?

BMI is not suitable for:

  • anyone with a heart condition that affects the fluid balance in their body, such as heart failure
  • people with lots of muscle
  • women more than 10 weeks pregnant 
  • children and teenagers aged 2-19, as growth charts should be used instead
  • babies and toddlers under 2 years.

What is my BMI?

Enter your height and weight

BMI categories

Your BMI result is best interpreted by a health professional who has more information about your other heart disease risk factors and overall health.

Generally, a BMI of:

  • less than 18.5 is below the normal BMI range
  • 18.5-24.9 is a normal BMI range
  • 25 or more is above the normal BMI range.

BMI is unable to measure how healthy you are. For this reason, the labels of ‘underweight’, ‘overweight’ and ‘obese’ can be unmotivating for some people and not effective in motivating positive behaviour change.

Manage your weight

Managing your weight is important for your health. When it comes to heart health it's best to focus on the things you can control in your everyday life.

These include:

Items depicting a health fitness lifestyle, including hand weights, trainers, an apple and a bottle of water.

Be aware of any sudden, unplanned and unexplained weight gain or weight loss. If this happens, it's best to speak to a health professional to find out why it’s happened.

If you're worried about your weight, we recommend discussing it with your GP, practice nurse or a registered dietitian or nutritionist.

 

What is my risk of heart disease?Manage your weight

References

1. Ross R et al (2020). Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity. Nature Reviews Endocrinology; 16(3):177-89.
2. Powell-Wiley TM et al (2021). Obesity and Cardiovascular Disease: A Scientific Statement From the American Heart Association. Circulation; 143(21):e984-e1010.