I am a registered nurse. I want to practice what I preach. Questions are welcome!

About Obesity, Overweight, BMI, and Body Fat Percentage

Knowledge is power. Cliche, maybe, but true. As you learn more about a problem, your ability to deal with that problem will increase. In most cases with health problems, your motivation to deal with the problem will also increase. So let’s start with some basic information for those who are new to weight loss or new to the topic of obesity and overweight as they relate to health.

Body Mass Index (BMI) is weight in pounds x 703 divided by height in inches squared (There is also a formula for the metric system). A BMI of 18.5 to 25 is considered normal, and less than that is underweight. Overweight is a BMI of 25-30, while obesity is a BMI > 30.

Obesity is then divided into three classes (here I’m getting data from the World Health Organization. Class I Obesity is a BMI of 30 - 34.9, Class II Obesity is 35 - 39.9, and Class III Obesity is a BMI at 40 or above (this is often referred to as “morbidly obese”).

In medicine, overweight is excess fat at a level associated with an increased risk of certain diseases in the long run, and is considered “not ideal”, whereas obesity is the point at which the person is likely to be experiencing some degree of damage to his or her health right now. Obesity is also associated with a high to very high risk of certain diseases (to be listed in a separate post).

BMI is not the entire picture, however. It is meant to be a rule of thumb and is not perfect. It is not a measurement of body fat percentage, nor does it account for lean muscle mass. Back when Arnold Schwarzenegger was better known for lifting weights than for governing California and fathering a love child, he was 6’2” and weighed 250 pounds.

Let’s do the BMI math:

       250 x 703/74 x 74 = 175750/5476 = a BMI of 32.09.

So, when Arnold was at the peak of his career as a bodybuilder, his height and weight put him in the category of obesity. Of course, most people are not bodybuilders, and this is an extreme example, but it illustrates that a mathematical formula based only upon a person’s height and weight can be an iffy means of classification. A much better measurement is body fat percentage.

These are the levels of body fat in each gender:

              Women:                                   Men:

Essential fat:  10-12%                      2-4%

Athletes:        14-20%                      6-13%

Fitness:          21-24%                      14-17%

Acceptable:     25-31%                      18-25% 

Obese:           32% and over              26% and over

Essential fat is the bare minimum amount that your body needs before it begins to malfunction. The athletic range is the amount typical for serious athletes. The fitness range is for people who are active and in good physical condition. Acceptable range is in between being in shape and being obese, and then you have the obesity category.

Bodybuilders like the “obese” Arnold Schwarzenegger have very low body fat percentages, usually between 4 and 10.

So why do we use BMI? Why don’t we just go with our body fat percentage? Well, the problem is that it’s difficult to measure body fat percentage in a way that is affordable, easy, portable, and accurate. You can get calipers online, but the high quality ones are expensive and they are easy to misuse. Also, in order to use them correctly, you need to have someone else helping you with the measurements. You can also have a body density test done that involves being weighed while in a water tank. Obviously, this isn’t exactly convenient, and it can be expensive depending on where you have it done. Another way of measuring body fat that is gaining in popularity is the use of a body scan called DEXA. The equipment for this method is expensive. Several other methods also exist, each of which has pros and cons, and none of which are as easy as calculating BMI.

I’ve looked around, and I like the concise, easy explanations given on this page:


About body fat measuring bathroom scales - The bad news is that they aren’t accurate in giving you an initial body fat percentage reading. The good news is that they are somewhat helpful in detecting changes in your body fat percentage (though I would be very wary and do lots of research on different models before buying one). So, if you begin losing weight, these scales can be helpful in tracking the amount of body fat you’re losing. Still, take the readings with a grain of salt.

Wouldn’t it be nice if calculating body fat percentage were as easy as calculating BMI?

  1. tobeahealthynurse posted this