Body mass index (BMI) is a simple index of weight-for-height that is used as a measure to determine if you are a healthy weight for your height. The BMI is basically a relationship between your weight and height that is associated with body fat, nutritional status and health risk. It is designed for men and women over the age of 18.
Try our BMI Calculator to calculate your healthy weight range
BMI is defined as the weight in kilograms divided by the square of the height in metres (kg/m2).
For example, an adult who weighs 60kg and whose height is 1.65m will have a BMI of 22.1.
Classification of adult underweight, overweight and obesity according to BMI
| Classification | BMI(kg/m2) | |
| Underweight | <18.50 | |
| Severe thinness | <16.00 | |
| Moderate thinness | 16.00 – 16.99 | |
| Mild thinness | 17.00 – 18.49 | |
| Normal range | 18.50 – 24.99 | |
| Overweight | ≥25.00 | |
| Pre-obese | 25.00 – 29.99 | |
| Obese | ≥30.00 | |
| Obese class I | 30.00 – 34.99 | |
| Obese class II | 35.00 – 39.99 | |
| Obese class III | ≥40.00 | |
Table adapted from the World Health Organisation
BMI values are age-independent and the same for both sexes.
Who can use the BMI?
BMI is designed for use in adult men and women over the age of 18. It is important to note that there are some individuals for whom the BMI is not an accurate measure, including pregnant women and athletes. This classification system is not suitable for children either, as they are still growing and for this reason BMI is plotted on growth charts.
What are some of the limitations with BMI?
BMI is useful for the general population as a measure of the best weight for health. However limitations include:
- Overestimating body fat in athletes and people with a muscular build
- Factors like ethnicity can influence the relationship between BMI and body fat
- BMI does not indicate where fat is being stored – fat stored between organs (referred to as “visceral fat”) can have significant health implications
Are there other measures of body fat available?
There are other measures to determine body fat such as skinfold thickness, BIA (Bioelectrical impendance) and underwater weighing. However these measures are often expensive, impractical and not widely available, meaning that waist measurements are a good tool. Waist measurements are a good way to determine where body fat is being stored.
What do the waist measurements mean?
The following waist measurements suggest an increased risk of health problems:
| Increased Risk
Men: more than 94cm Women: more than 80cm |
Greatly increased risk
Men: more than 102cm Women: more than 88cm |
This article was written by our dietitian Rhiannon Welsh who is an Accredited Practising Dietitian and Dietitians Association of Australia member.
If you have questions about body mass index or healthy eating, contact your local doctor who will arrange for you to see a dietitian in Sydney. Contact us today!
References for further information
