Body mass index (BMI) is a simple tool that is generally used to estimate the total amount of body fat.It was first invented in the 19th century by a Belgian polymath called Adolphe Quetelet, hence it is also known as Quetelet Index.BMI gained popularity as a body weight-tracking device in the mid twentieth century. This coincided with the time when obesity began to raise its ugly head in the more prosperous west. It became a tool whereby a doctor could actually guage a patient being too heavy or too thin and even discuss the issue with the individual. To calculate BMI, you need to know your weight and height in kilo-grams and centimeters or meters. The weight of a person is then divided by the height.