by Anne Finch, Accredited Practising Dietitian for LiveLighter

What is BMI?

Body mass index (BMI) is a simple tool for classifying a person's weight. It’s calculated by looking at the relationship between height and weight to provide an indication of whether a person is underweight, at healthy weight, overweight or obese.

This is important because we know that carrying too much extra body fat increases the risk of health problems, like type 2 diabetes, heart disease, fatty liver disease and certain cancers.

It’s important to remember that not that everyone with a high BMI is unhealthy, and not everyone with a “healthy” BMI is healthy. Eating well and being active is great for your health regardless of your weight or shape.

How can I find out my BMI?

You need to know your height (in metres) and weight (in kg). You can use an online calculator to do the maths for you or use this formula:

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You can do this on a normal calculator by typing your weight (in kg) then pressing ÷ and typing in your height (in metres). Then press ÷ again and enter your height again. Then press =

Eg. if someone is 82 kg and 175 cm (1.75 m) tall they would press

82 ÷ 1.75 ÷ 1.75 =

26.77

They have a BMI of 26.77

BMI classifications

The World Health Organisation uses these classifications:

BMI                   Classification
Under 18.5 Underweight
18.5 - 25            Healthy weight
25 - 30             Overweight
30+ Obese

For some groups of people, the healthy BMI range might be higher (as for older people and people of Pacific Islander background) or lower (as for people of Asian or Aboriginal background).

Issues with BMI

High BMI isn’t the same as too much body fat – it’s a simple formula that can make a good guess. The calculation can’t tell the difference between someone who has a lot of muscle, and someone who has a lot of fat. It also can’t tell the difference between someone who has fat around their belly (worse for health) and someone who has fat around their hips and bottom (not as bad for health).

Be careful interpreting your BMI result if you belong to one of these groups – it’s less accurate for

Body builders and weight lifters
Some high performance athletes
Pregnant women
People over 65
People with a physical disability
People with Polynesian, Pacific Island, Aboriginal, South Asian, Chinese and Japanese heritage
Note that this BMI calculation is not suitable for children. Use this calculator for people 2 -20 years old.

BMI is still useful

BMI is not a perfect measure of body fat or health risk – but it’s a good place to start. It’s used a lot because most people know their height and weight. There are better ways to measure body fat, but they can take a long time, need trained staff or special equipment.

We recommend looking at BMI and waist measurement together. If someone has a high BMI because they are very muscly, or has fat on their hips and bottom, they will probably have a smaller waist measurement. Having a big waist measurement is actually a better predictor of future disease risk than BMI. It is used less often because not everyone knows their waist measurement.

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In conclusion…

BMI is a simple tool that can be useful to give you an idea about your health risks – but it’s not perfect! Waist circumference is a slightly better tool, and you can look at both of these measurements together. To get a better look at your overall health picture, see your doctor. They can look at other health markers like blood pressure and cholesterol and give you advice that is specific to your body and health.