The Body Mass Index (BMI) is a calculation of the ratio between your weight and your height. The formula is used to determine the amount of body fat you carry. The formula is your weight divided by your height squared (BMI = kg/m2)
Q. What is BMI designed to do?
A. BMI is used as a simple tool used to estimate body fatness. However, we need to remember it is not directly measuring fat, as it only takes into account height and weight. It is a useful screening tool to identify whether or not a person might have a weight-related health issue.
Q. What are the limitations of the BMI?
A. There is a wealth of research showing that BMI correlates well to body fatness in large groups of people. It is less reliable for any one individual. Often BMI is used as an initial screening tool to identify people who may have health problems associated with being under- or overweight, but it is not a diagnostic tool. Other measures could then be used to see whether or not that individual had any health issues that may be related to their weight. If you don't think the BMI is useful for you, then we suggest it's best not to use it. If you are concerned about your weight, it's a good idea to raise this with your GP, or see a dietitian or nutritionist, to discuss your individual situation.
Q. Who shouldn't use the BMI
A. Athletes with higher amounts of muscle mass, women who are pregnant or lactating, growing children and young people, and the elderly who have lost muscle mass are advised not to use the BMI.
Q. Why should we care about body fat anyway?
A. Excess body weight is one of the most important risk factors for diseases that are killing us that we can actually change. Overweight or obesity often, but not always, results in high blood pressure, high cholesterol and abnormal blood glucose responses, which in turn are key in the development of heart disease, stroke, several common cancers, as well as type 2 diabetes. And while they might not kill you, being overweight or obese also makes it more likely you'll suffer from osteoarthritis, gout, gallstones, sleep apnoea and decreased fertility. This is not an exhaustive list, but you get the general picture: being overweight or obese is not good for us. Remember too that the cut-offs are only arbitrary guides. Your health risk doesn't suddenly increase dramatically when you go from a BMI of 29 to a BMI of 30, for example. It actually works on a continuum, so that as your BMI increases past the healthy range (or decreases below it), your risk of health problems increases too.
Q. Can the BMI measure my fitness level?
A. No. And if you are overweight as measured by the BMI, being fit and overweight is much healthier than being unfit and overweight.
Q. Are there other measures of body fatness we could use?
A. There are two simple measures that have been shown to also be good indicators of abdominal obesity and its accompanying health risk. The first is waist circumference. The waist is halfway between the bottom of the ribs and the top of the hips (usually around the belly button). For bigger people, measure the waist at the widest point. Again, there's some debate about what the actual cut-off points should be, but to a certain degree this is just semantics; what you need to understand is your health risk increases as your girth increases.
80cm or more
94cm or more
88cm or more
102 cm or more
The second is the waist-to-height ratio. It's easy to remember the guideline: your waist circumference should not exceed half of your height. This applies to both men and women. So for example, if you are 170cm tall, your waist circumference should be no more than 85 cm.
If this is your height in metres...
...your waist should be equal to or less than this in centimetres
While there is still a lot of debate about which measure most accurately predicts risk, it has been shown that each of these three measures – BMI, waist circumference, and waist-to-height ratio – are all useful guides.