We've all been told about the dangers of being too fat or too thin, but how do you tell if your size is putting your health at risk or how much you should really weigh? We look at some of the best ways of assessing whether it's time to take action.

Being too thin or too fat is unhealthy, but good health is about much more than just your weight. It depends on many things, including your family's medical history, your genes, whether you smoke, the type of food you eat and how active you are. It's therefore difficult to set an exact ideal healthy weight that applies to everyone.

Many people have a distorted perception of what constitutes a healthy body weight

Many people have a distorted perception of what constitutes a healthy body weight. We're surrounded by images of glamorous people, many of whom are in fact underweight. Looking at yourself in the mirror isn't a very useful way to assess whether you're a healthy weight either. Most people compare themselves with others rather than by objective standards.

Doctors check your body size by measuring your weight and height. This calculation is known as the body mass index (BMI). Research has shown that people within a certain range of body size tend to live the longest and enjoy the best health.

You can check whether you have a healthy weight for your height using our body mass index calculator, available in both metric and imperial versions.

Another method of assessing whether you're a healthy weight is to measure your waist.

It's important to remember, however, that both these are only one measure of your health. Body fat percentage, blood pressure, resting heart rate, cholesterol and other measurements are at least as important. You need to get the whole picture.

We all know that being overweight can put your health at risk, but it can also suffer if you weigh too little.

If you're underweight because of a restriction of your diet, you're at risk of a number of nutritional deficiencies.

Young women especially are at risk of anaemia - a lack of iron - while insufficient calcium can lay the foundation for osteoporosis later in life. Amenorrhoea (missing menstrual periods) is also common among women who are underweight and can lead to infertility.

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