BMI, or Body Mass Index, is a measure of a person's body fat, based on a simple calculation of body weight in kilograms, divided by the square of the individual's height in meters.

It is used by health professionals around the world to determine whether a person is overweight or clinically obese. A person with a BMI equal to or over 25 is considered overweight, while an index of 30 or more is considered obese. At the opposite end of the scale, a BMI of less than 18.5 is an indication that a person is underweight. The optimum, healthy range is between 18.5 and 25.

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Because BMI doesn't take any factors other than height and weight into consideration, many people argue that it is far too simplistic. A frequently quoted example of where it is likely to be misleading is with regard to muscle mass

Since muscle weighs more than fat, people who have a higher percentage of body muscle, such as athletes, could, on a strict interpretation of their body mass index, be considered obese a classification that for most professional athletes would clearly not be the case.
Others say that the health risks associated with a high BMI decrease with age, so that it is not an appropriate health indicator for older people. Increasingly, experts are reporting that alternative calculations, such as waist circumference or waist-to-hip ratio, should be used as more accurate predictors of a person's risk of developing serious health problems.

That's because the focus for these calculations is on where the excess body fat is actually being stored -- the argument being that fat located on the hips, for instance, is not as bad for you as fat located around the stomach.

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These arguments notwithstanding, BMI is nevertheless a useful, if imprecise, indicator of general health. With the World Health Organization estimating that at least a third of the world's adult population is overweight, and nearly a tenth is obese, the importance of finding and maintaining a healthy lifestyle is being brought more and more to the forefront.

There are numerous incentives for ensuring you keep your BMI within the healthy range. Perhaps the most important of these is the fact that you will be at a much lower risk of heart disease, diabetes, stroke, and certain cancers. Other conditions linked to obesity are joint problems, back pain and sleep deficiency.

Studies have also linked a high BMI with decreased fertility in women. In addition, many people who are overweight also suffer from problems such as depression and low self-esteem. In other words, keeping an eye on your weight and therefore on your BMI - will not only help you look better, you're also more likely to feel much better too.

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In simple terms, the way to ensure you maintain a healthy weight - without resorting to elaborate or faddy diets - is to burn more calories than you consume. This is ideally done through a combination of balanced diet and an active lifestyle, which are both best achieved, and more importantly, sustained by incorporating them into the routine of your everyday life.