BMI and waist measures

Body mass index

Working out BMI

BMI is a number calculated from your weight and height, healthdirect says. You work it out by dividing your weight by the square of your height, that is, your height multiplied by itself:

healthdirect also offers a BMI calculator, linked from its BMI and waist circumference page.

Classifications

The four adult ranges

healthdirect uses 4 weight classifications, and for adults gives these BMI ranges:

healthdirect describes overweight and obesity as health conditions in which body weight is higher than is good for health. healthdirect calls BMI a guide to whether you are a healthy weight for your height, and says a high BMI can mean a higher chance of getting some chronic conditions.

Limits

Where BMI is less accurate

BMI is not always accurate for showing whether you are a healthy weight, healthdirect says, and how useful it is can depend on your body shape, age, sex or ethnic origin. It names groups of people in whom BMI is a less accurate test:

• some high-performance athletes
• bodybuilders or weightlifters
• women who are pregnant
• people with a physical disability
• people under 18 years.

healthdirect adds that BMI is also less accurate in people from some ethnic groups: Aboriginal and/or Torres Strait Islander peoples, and Pacific Islander, Maori, South Asian, Chinese and Japanese people.

Waist circumference

Measuring the waist

The correct place to measure, healthdirect says, is halfway between your lowest rib and the top of your hipbone, roughly level with your belly button. Generally, the larger the waist measurement, the more chance there is of developing health problems.

healthdirect gives these waist measurements as ones that can show an increased risk of disease:

Waist measurements only apply to adults, healthdirect says. They do not apply to children, pregnant people, people with a health issue where the abdomen (tummy) is very enlarged, or people from certain non-European backgrounds, who may have a different body shape. Its page on internal body fat adds that measuring the waist is not suitable for everyone.

Internal body fat

The waist and internal body fat

healthdirect calls measuring around the waist the main way to see whether you have too much internal body fat, the fat on and around the organs. It says your risk of storing extra energy as internal fat may be greater if you:

• are an older man
• are a woman past menopause
• were of low weight at birth
• are genetically predisposed
• drink too much alcohol.

That is healthdirect’s list in full; its page on internal body fat gives the context. The same page says excess internal fat forms when you take in more calories than you burn and do not move enough, and that too much of it around the belly can be a sign of insulin resistance and metabolic syndrome. Measuring your BMI may also help show whether you are in a healthy weight range for your height, the same page says.

Scan comparison

BMI, the waist and a DXA scan

A DXA scan, as ARPANSA describes it, can derive fat mass and lean soft tissue mass among its measures. ARPANSA’s committee also says that scanning for those, with no clinical indication behind the scan, does not justify the radiation. DXA and radiation sets out its statement, and What a scan measures lists the measures.

Advice

Someone to talk to

If you are worried that you may be carrying an unhealthy amount of internal body fat, healthdirect says to talk to your doctor. healthdirect’s helpline is 1800 022 222, called NURSE-ON-CALL in Victoria, and a registered nurse is there to speak with you 24 hours a day, 7 days a week.