BMI, or Body Mass Index is a method of determining if a person is overweight based on a realtionship between their height and weight. It is determined by dividing weight by height squared. For example, 80kg divided by (1.8m X 1.8m). This will give you a number that corresponds to a position on the BMI chart.
It is important to point out that this should only be used as a PRELIMINARY method of determining whether or not a person is overweight. BMI does not take into account frame size, muscle mass or age. For instance, a person that is very muscular will have a higher weight to height ratio than a person who does not work out. This may make them appear overweight on the BMI chart when they are anything but.
It is best not to fixate on the BMI chart, or better yet, ignore it. The best way to determine if you are losing weight is if your clothes fit better and you feel lighter. BMI is only for reference and is not the most accurate tool for determining over or underweight. It is based on the “average” person and each individual is anything but “average”. Use a tape measure and measure key points on your body once a month. As long as the measurements are dropping, you are losing weight. Even the scale may tell a different tale – as you exercise you will build muscle which is leaner and heavier than fat. In other words, as you exercise, your weight may go up as your body size shrinks (loses fat). This again will throw off the BMI chart so it is important not to rely on it too much.