Body mass index

Adult weight and height

In the UK, there have been increasing concerns about how people are getting heavier and the resulting problems of public health. The International Obesity Taskforce reports that this is particularly a problem in England where obesity has risen threefold between 1980 and 2001. Understanding healthy body weight has probably never been more important.

Body weight and health

A number of serious health problems are known to be associated with a high Body Mass Index (BMI). In Western countries it is estimated that 90% of type 2 diabetes cases are caused by excessive weight. Being overweight significantly increases the risks of heart disease or strokes.

In October 2007 a major report from the World Cancer Research Fund (WCRF) identified keeping a healthy body weight (BMI of 20-25) as one of the most important things that can be done to prevent cancer.

Knowing your weight in kilograms and height in metres makes it simple to calculate your Body Mass Index. Using metric helps you to understand how healthy your weight is.

Where does the BMI come from?

The Body Mass Index was developed by the World Health Organisation and allows people to quickly and easily establish whether they are under weight, normal weight, overweight or obese.

It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70 kg and whose height is 1.75 m will have a BMI of 22.9.

BMI calculation

Calculating a BMI score

First, make sure you know your weight in kilograms (kg) – if you don’t know your weight in kg, there is advice here – and your height in metres, which you can calculate here. Make a note of your height in metres and weight in kg, so that you can easily monitor your BMI in future.

Then use a calculator or a website with a built-in BMI calculator to easily work out your score such as this health information web site

What is a Healthy BMI?

Generally speaking, the following applies for understanding your BMI. If it is

  • Under 20 you are underweight
  • Between 20 and 25 you have a normal weight
  • Between 25 and 30 you are overweight
  • Between 30 and 35 you are obese
  • Above 35 you are clinically obese.

BMI chart

Some people have pointed out that a few very fit sports players, e.g. international rugby players, may have a BMI that is around 30 and be very healthy. However it should be obvious whether you have a BMI of 30 through muscle bulk or obesity. BMI remains a simple, accurate indication of the health of body weight for the overwhelming majority of people. As WCRF’s report says “the BMI method provides a simple measure of body fatness for most people, most of the time”

If you are overweight, how much weight should come off?

Suppose you are 1.8 m tall and weigh 90 kg; your BMI is 27.7 which is overweight. To get to a healthier weight you would probably choose to increase your exercise and improve what you eat. But what weight should you aim for?

Suppose you decide that you want to get to a healthy BMI of 22. You need to get to

(22 / 27.7) x 90 kg = 71.5 kg

Try working that out if you start with your weight at the equivalent 14 stone 2 pounds 6 ounces.

If you work in kilograms you can very easily work out your percentage gain or loss in weight.