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.

The BMI ranges are as follows:

IMPORTANT: Please note that BMI is not as accurate if you are an athlete or very muscled (muscle weighs more than fat), as it can push you into a higher BMI category despite having a healthy level of body fat. It is also not accurate for women who are pregnant or breastfeeding, or people who are frail.

The BMI Calculator is only one guide about your overall health. Waist measurement, body fat level, blood pressure, cholesterol, physical activity, not smoking and the healthiness of your diet are also important. You need to get the whole picture.

Another method of assessing whether you're a healthy weight is to measure your waist. A measurement of more than 94cm (37in) for men and 80cm (32in) for women suggests you should try not to gain any more weight. An extra 8cm (3in) is an indication that your health will probably improve if you lose a little weight.

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.

If your weight lies within the healthy range but you're unhappy with your shape, you're likely to derive more benefits from a supervised exercise programme than by restricting your food. This will improve your fitness, help to tone specific muscle groups and enhance your overall health and wellbeing. Plan your diet to optimise your health.

This article was last medically reviewed by Dr Rob Hicks in October 2005.
First published in May 2001.