Body mass index (BMI) is used as an estimate of total body fatness that is a useful measure provided that age and sex are taken into account. This is because weight differences among individuals of a similar age and sex is normally due to body fat. The exceptions to this rule are body builders, pregnant women, patients with anorexia nervosa, and those with massive obesity (1).

The formula for calculating BMI is:

BMI = weight (kg)/height (m²)

A BMI of between 20 to 25 corresponds to the healthy weight range for adults (18 years or over) Under 20 indicates underweight, over 25 indicates overweight. There is an increased rate of mortality for individuals above or below this healthy weight range. Underweight individuals have increased risk for respiratory disease, tuberculosis, digestive disease and some cancers. Overweight individuals are more prone to cardiovascular disease, gallbladder disease, high blood pressure and diabetes (1).

BMI is useful because it is easy to calculate, but it is not a particularly accurate measure because it does not differentiate muscle mass from fat mass. BMI can only be used to determine body fatness in adults. In children, it is more appropriate to use growth charts to measure the gain in height and weight (2).

Calculate your body mass index on-line.

References

1. Forbes, Gilbert, B. Body Composition. in: Ziegler E.E., & Filer, Jr., L.J. [ed], Present knowledge in Nutrition. Washington DC: International Life Sciences Institute Press.  1996, pp7.

2. Read, R.S.D., Kouris-Blazos, A.. Overweight and Obesity. in: Wahlqvist, M.L. [ed], Food and Nutrition - Australiasia, Asia & the Pacific. Melbourne: Allen & Unwin Pty Ltd. pp348, 1997.