Although a person's BMI does not provide clear measure of exactly how much body fat is present in an individual, studies show that people with higher levels of body fat will also have higher BMI figures, making it sufficiently reliable in most circumstances.
A 'normal' BMI value is between 18.5 and 24.9. A person with a BMI value below this range would be considered underweight, while anyone with a figure higher than 24.9 would be considered overweight.
An individual with a BMI of more than 30 would be classified as obese and therefore at increased risk of weight-related health problems, while anyone with a value higher than 40 would be considered extremely obese and advised to take immediate action to reduce their weight.
The higher your BMI figure, the more likely you are to develop weight-related health problems such as high blood pressure, diabetes, heart disease, and certain forms of cancer.
The idea that the ratio of a person's height and weight could be an indicator of health was first developed by Belgian statistician Adolphe Quetelet in 1832. Initially known as the Quetelet Index, the system was adopted by the World Health Organization in 1995 and renamed the Body Mass Index.
Although BMI provides a broad indication of the level of body fat in most individuals, it does have a number of limitations. Most crucially, BMI is not able to account for muscle mass and, as muscle tissue is denser and therefore heavier than fat tissue, certain individuals can receive results indicating they are overweight, despite being in excellent health.
In particular boxers, weight lifters, rugby players, and other athletes with low body fat levels and large amount of muscle will often register readings suggesting they are overweight. One study of gold medal winners at the 2004 Olympics in Athens found at least 15 had such high BMI figures that they would actually rank as obese.
Age and gender also play a role. If a woman and a man have the same BMI, the woman will usually have a higher proportion of body fat. Similarly, an older person with a particular BMI will generally have a higher proportion of body fat that a younger person with the same BMI figure.
BMI also fails to take into account the way fat is distributed around a person's body. Someone with a large amount of excess weight around their waist will considerably be less healthy than someone whose excess weight is spread evenly over their entire body.
However, two such individuals could have identical BMI figures - despite having completely different body shapes and therefore different levels of health risk.
The best way to approach BMI is to see it as just one of a number of measures that you can use to assess the state of your health.
Written by Dan Richard, a sports and gym enthusiast writing about weight loss.
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