BMI stands for Body Mass Index and is a system used to determine whether a patient is overweight or obese. It is calculated by dividing a person's weight by their height. As an example, a woman who is 5'6" and weighs 10 stone has a BMI of 22.6.
Adults can use the NHS Direct BMI calculator to get a personal reading, but BMI is less easy to establish in children because their reading will change as they continue to grow. Adults should aim for a healthy number which lies between 18.5 and 24.9. Above this means you're at risk of developing serious conditions such as heart disease, diabetes, stroke or cancer. The World Health Organisation (WHO) says that a BMI calculator reading of between 25 and 29.9 indicates you're over the recommended weight for your height, a reading of between 30 and 39.9 means you're obese, and a reading over 40 means you're very obese.
Obesity is a serious issue and levels are rising each year. A 2005 Health Survey for England reported that over 21 percent of men and women were classified as obese and, more recently, research from Foresight, a Department of Health sponsored organisation looked into the causes of obesity, finding that weight-gain isn't just about over-indulgence or laziness.
The report suggests that the technological revolution of the 20th century has led to unavoidable weight gain for many of the population because our bodies and biological make-up are out of synch with our surroundings. If current obesity growth rates continue, it has been predicted that some 60 percent of men, 50 percent of women and 25 percent of children in Britain will be obese by 2050.
Depending on your BMI reading, you can establish whether you need to make subtle - or serious - lifestyle changes for your long term health. However, remember the BMI test doesn't take into account a person's body fat content, so your BMI calculator reading may not be accurate if you're a weight-trainer, athlete or just very muscular because muscle weighs more than fat.
Consequently, you could have a higher BMI reading regardless of a healthy body fat level. BMI testing may also not be accurate if you're pregnant or breastfeeding and if you're over the age of 60 because bones weigh less as you get older.