Understanding your BMI (body mass index)

What is the body mass index?

Body mass index (BMI) is a measure to decide if your weight is reasonable for your height. Your BMI is your weight in kilograms divided by your height in metres squared.

What is a normal BMI?

According to the World Health Organisation (WHO), a healthy BMI is between 18.5 - 24.9. See the chart below.

Extra fat is the commonest cause of a high BMI. If you have a BMI of over 25, you might start thinking about making some lifestyle changes. However, this may not apply to pregnant women, children, older people and athletes, or very muscular people.

If your body mass index is obese (BMI 30-39.9) or morbidly obese (BMI over 40) then your risk of health problems is high. It is important that you try to lose some weight and should consider asking for help from your doctor.

It should be noted that BMI does not take into account body composition, or the distribution of fat about the body. Your body is made up of two components, lean body tissue and fat. The relative proportions of these determine your body composition.

Body fat percentage

A certain amount of body fat is essential for good health. Health clubs or weight management clinics may use a body fat analyser to measure your body fat. The table below is a guide to healthy body fat percentage.

In addition to the total amount of body fat you carry, there is evidence to suggest that the distribution of fat about the body is related to health risk. Fat distribution can be divided into two types - apple-shaped and pear-shaped.

Apple shaped is the central or abdominal fat stored around the abdomen, typically seen in men. Pear-shaped is peripheral fat distributed around the hips and thighs, most commonly seen in women. It is thought that men and women with central obesity (fat around the mid and upper parts of the body) are at a greater risk of developing heart disease than those whose fat distribution is around the hips and thighs.