Health professionals use the BMI calculator and other tools to measure and categorise a person’s weight into obese, overweight, healthy or underweight.
On this page:
- How body fat is measured: the BMI calculator
- What to do if you’re underweight
- What obese and overweight mean: how obesity affects you and what you can do
Most doctors and health practitioners in New Zealand use a simple measurement called the Body Mass Index (the BMI) to estimate the level of fat in the body.
The BMI is calculated using your height and weight data.
Healthy weight: For adults, a healthy weight is a BMI measurement between 18.5 and 24.9 kg/m2.
Overweight: For adults, a BMI measurement of 25 kg/m2 or over is considered overweight.
Obese: For adults, a BMI measurement of 30 kg/m2 is considered to be obese.
Underweight: For adults, a BMI measurement of 18.49 kg/m2 or less is considered to be underweight.
Your risk of developing health problems increases as your BMI gets higher above, or further below, the 'healthy weight' range.
BMI is a simple easy to use measure of excess weight for height. However, it is not able to differentiate between body fat and muscle mass.
Some highly muscular people may have a high BMI because muscle is heavier than fat. In addition, BMI, and definitions of overweight and obese based on BMI, may not be appropriate for people, particularly children, with certain health conditions such as some endocrine or genetic conditions.
If you think you’re pregnant then refer to the advice in Healthy weight gain during pregnancy.
Waist circumference can be used along with BMI to further assess weight status and disease risk.
You can calculate your BMI using the Healthy weight calculator tool.
Being underweight can increase your risk of, or be a sign of ill health. Talk to your doctor or practice nurse for further advice.
Overweight and obese are medical terms used to describe a person who has accumulated body fat at a level above what is considered healthy.
BMI provides an indication of excess weight for height but is unable to differentiate between fat and muscle mass.
A doctor may also measure waist circumference to assist in determining health risk due to excess weight.
How obesity affects you
Being obese is not good for your health. It can increase your risk of developing conditions and serious diseases including:
What you can do
An unexplained change in weight (increase or decrease), particularly if over a short period, can be a sign that you have an underlying health condition. If so, it is important that you see your doctor.
It is also important to consider other factors that may be influencing your eating and activity patterns. Read more about the other influences in What affects your weight
Try to identify the positive things that help you to eat more healthily such as supportive friends and whānau. Identifying and addressing the underlying factors, will help create a more supportive environment for you to eat more healthily, be active and get enough sleep.
We recommend the FAB approach, Food, Activity (including reducing sitting time, and getting enough sleep), and Behavioural strategies.