BMI - Body Mass Index

What is BMI?

BMI, Body Mass Index, is a tool used to determine whether you are in a healthy weight range for your height.

How do I calculate my BMI?

Click here to use the Heart Foundations online BMI calculator

What BMI means

If you have a BMI of:

  • Under 18.5 – you are considered underweight and possibly malnourished.
  • 18.5 to 24.9 – you are within a healthy weight range for young and middle-aged adults.
  • 25.0 to 29.9 – you are considered overweight.
  • Over 30 – you are considered obese.

For older Australians over the age of 70 years, general health status may be more important than being mildly overweight. Some researchers have suggested that a BMI range of 22-26 is desirable for older Australians.

Some exceptions to the BMI rule

BMI does not differentiate between body fat and muscle mass. This means there are some exceptions to the BMI guidelines, including:

  • Muscles – body builders and people who have a lot of muscle bulk will have a high BMI, but are not overweight.
  • Physical disabilities – people who have a physical disability and are unable to walk may have muscle wasting. Their BMI may be slightly lower, but this does not necessarily mean they are underweight. In these instances, it is important to consult a dietitian who will provide helpful advice.
  • Height – BMI is not totally independent of height and it tends to overestimate obesity among shorter people and underestimate it among taller people. Therefore, BMI should not be used as a guide for adults who are very short (less than 150 cm) or very tall (more than 190 cm).
  • People of different ethnic groups – Asians and Indians, for example, have more body fat at any given BMI compared to people of European descent. Therefore, the cut-offs for overweight and obesity may need to be lower for these populations. This is because an increased risk of diabetes and cardiovascular disease begins at a BMI as low as 23 in Asian populations. Some populations have equivalent risks at a higher BMI, such as people of Torres Strait Islander and Maori origin.

Being overweight or underweight can affect your health

The link between being overweight or obese and the chance you will become ill is not definite. Research is ongoing, although statistically, there is a greater chance of developing various diseases if you are overweight. For example, the risk of death rises slightly (by 20 to 30 per cent) as BMI rises from 25 to 27. As BMI rises above 27, the risk of death rises more steeply (by 60 per cent).

 

Risks of being overweight (high BMI) and physically inactive

If you are overweight (with a BMI over 25) and physically inactive, you may develop:

  • cardiovascular (heart and blood circulation) disease
  • gallbladder disease
  • high blood pressure (hypertension)
  • type 2 diabetes
  • osteoarthritis
  • certain types of cancer, such as colon and breast cancer
  • depression and other mental health disorders.

Risks of being underweight (low BMI)

If you are underweight (BMI less than 18.5), you may be malnourished and develop:

  • compromised immune function
  • respiratory disease
  • digestive diseases
  • cancer
  • osteoporosis.