BMI stands for Body Mass Index. It is a commonly used method of determining whether an individual is underweight, normal, overweight, obese, or extremely obese. It is nothing more than a special number that is calculated based on your height and weight (and nothing else), which you then look up on a chart. Your doctor may refer to it if you discuss your weight with him/her. It is almost certain that your health insurance company uses it. Even the Wii Fit uses it. This article discusses the steps to calculating your BMI, we well as more practical and accurate steps to take to determine if you are overweight or obese.
For those of you who like math, you can compute your BMI by taking your weight(in pounds), and dividing it by the square of your height (in inches). Then take that quotient, multiply it by 703, and note the result. That's your BMI score. Alternatively, you can use a chart like the one pictured above to determine this number.
Some charts are color-coded to immediately show you the category of your weight (overweight, obese, etc). If your chart does not show this then it will likely have the following legend that you can refer to for your BMI score: Underweight (Under 18.5), Normal (18.5-24.99), Overweight (25-29.99), Obese (30 -40), Extremely Obese (40 and higher).
That's it. You now have your BMI score, and what is supposedly means. It is worth noting several things, in an attempt to answer the "and should you" portion of this article's title. First of all, the BMI score is based on nothing other than height and weight. It doesn't take anything else into account, such as age, gender, body frame, body fat percentage, or muscular development. Many gold-medal-winning Olympic athletes are considered obese according to their BMI.
As a more practical step, if you want to categorize your weight, simply take off all your clothes, and look in a full length mirror. Turn all around, look at yourself from any and all angles, and make an assessment. You don't need any charts for this. Unless you are totally delusional, and are totally fooling yourself, you will simply be able to see what the situation is. If you are a professional athlete who is slim and muscular, you will see that. If you have a big beer belly, you will see that as well. If you are underweight, you will see that. Just look, and make a plan of action as need be, working closely with your doctor. Just be honest with yourself. If you see a big belly in the mirror, just accept that it isn't healthy, but you can do something about it, unlike people who have incurable medical conditions.
As the final step, if you are overweight or obese, and feel the need to assign a concrete number to your condition, the best thing to do is have your body fat percentage calculated. You can get a rough idea of this by using a simple, low-cost device that sends a mild current through your body which you do not feel at all. You need to follow the instructions carefully, and use it under appropriate conditions to not get a false reading. It is still not totally accurate. You can have your percentage calculated more accurately by having a professional use skinfold calipers to measure it. Finally, you can have your percentage calculated by doing hydrostatic weighing. This requires a facility that has a special weight tank that you must immerse yourself in.
Keep in mind that if you are overweight, you will know if you are losing weight based on looking in the mirror, and of course by using a simple scale. You will also know it by the way that your clothes feel, or by how many notches on your belt you are using. We do not need to be so dependent upon special methods, especially oversimplified and generic charts, to tell us such obvious things about our health.