Calculating Body Composition

Diet and Weight Loss Tutorial

As explained in What It Takes to Lose It All, the goal of your diet and weight loss plan should not be to lose weight, but to lose body fat. Losing muscle results in a lower metabolism, the rate at which you burn calories, and can ultimately cause your weight loss plan to fail.

Muscle is healthy, makes you look better, and burns calories. It's a good thing. But a scale doesn't tell you whether you are losing body fat or muscle, so you could be losing muscle and not realize it.

A scale also doesn't tell you if you are gaining muscle. Again, muscle is good. But when you are on a weight loss program, gaining muscle can be both confusing and discouraging if you don't realize how it's affecting your weight.

The solution is not to judge your progress by weight alone, but to calculate the percentage of your weight that is body fat and make it your goal to reduce that number, not just your weight.

And once you have determined what percentage of your weight is body fat, it is easy to estimate your muscle mass so that you can monitor that, too, and ensure that you don't lose muscle along with the fat.

Mirrors Don't Lie

Measuring your weight on a scale is the worst way to track your progress. Daily fluctuations in the amount of water in your body, caused by the amount of salt you eat and other factors, cause your weight to vary. These fluctuations have nothing to do with permanent weight loss, and will only serve to frustrate you.

If you choose to use a scale to monitor your progress, it is generally advised that you not weigh yourself more than once a week. You should also weigh yourself first thing in the morning, when the water content of your body will be more consistent.

They say that mirrors don't lie, and a mirror is actually a great way to see changes in your body. The problem is that many of us are overly critical of ourselves and never seem to like what we see in the mirror. We always find something we don't like and dwell on it, so it helps to have something less subjective to go by.

Clothes don't lie either, and seeing how your clothes fit is a great way to observe the changes in your body. Or if you'd like to be a little more accurate and keep records of your progress, you can take measurements as described in our newsletter Measuring Success.

However, none of these methods makes it very clear whether or not you are losing muscle along with the fat. So here are some methods of calculating body fat percentage with which you can measure both fat and muscle loss, or gain.

Methods of Measuring Body Fat Percentage

The "gold standard" for measuring body fat percentage involves weighing your body in water. You sit on a seat that hangs from a scale and are submerged into water. Because fat floats and muscle sinks, the more body fat you have, the less you will weigh.

This process, called hydrostatic testing, if most often performed in hospitals and university labs. While it is the most accurate method, it is obviously not a practical means of monitoring your progress on a weekly basis.

There are many other methods of testing involving chemical analysis and electronic devices. But we will concentrate on the two methods that are most practical for monitoring your progress at home: Bioelectrical Impedance Analysis (BIA) and skinfold measurements.

Bioelectrical Impedance Analysis (BIA)

BIA sends a small electrical charge or current through your body and determines the percentage of body fat by measuring the resistance. The determination is based on the fact that muscle, with its high content of water, is more conductive than fat. Thus muscle tends to transmit the current, while fat impedes it.

If the test is performed in a doctor's office, it is likely that one electrode will be attached to your wrist and another to your foot. The BIA device measures how much current passes from one electrode to the other.

You can perform a similar test at home on a BIA scale with great convenience. You simply stand on the scale, barefoot, and the current passes from the electrode under one foot to the electrode under the other. Because the water content of your body affects the measurement, it is best to use the scale first thing in the morning.

The body fat percentage calculated by a BIA scale is not likely to be as accurate as the skinfold method described next, but monitoring the relative change can be very useful. Making the measurement is as simple as removing your socks, and the results are instant.

Pictured is a BIA scale manufactured by Tanita.

Skinfold Measurements

Skinfold measuring uses a simple pair of calipers to measure subcutaneous fat, which lies just beneath your skin. While this method is not as simple as standing on a scale, it is more accurate and can easily be done at home.

The method is to pinch a fold of skin between your thumb and index finger, and then use the calipers to measure the thickness of the fold. Repeating this process at several different locations increases the accuracy of the measurement.

The location(s) to measure and the formula for calculating body fat percentage will be included in the instructions for the calipers you purchase. You can also purchase electronic, digital calipers that do the math for you.

Pictured are two sets of AccuFitness calipers. The first set is manual; the second set is digital.

Ideal Body Fat Percentage

In his book Burn the Fat, Feed the Muscle, Tom Venuto writes that an optimal body fat percentage for a non-athlete man is around 10-14%, while it is around 16-20% for a non-athlete woman. These percentages should provide a lean appearance. The difference between genders is due to the fact that men are naturally more muscular (lean) than women.

Searching the web for recommended body fat percentages, you may find these numbers a little low. It depends on what you want for yourself. If you want a "ripped" look, you may want to go even lower. You will also find charts suggesting that the numbers increase with age. They typically do, but nothing says that you have to be typical.

The American Council on Exercise provides the following ranges for men and women with classifications based on varying levels of body fat:

Classification Men Women
Essential Fat 2-5% 10-13%
Athlete 6-13% 14-20%
Fitness 14-17% 21-24%
Average 18-24% 25-31%
Obese 25% and above 32% and above
Source: American Council on Exercise (2003) ACE Personal Trainer Manual, 3rd Ed., Ch. 6, Pg. 188, Table 6.14, General Body Fat Percentage Categories

Calculating Fat Weight and Lean Body Mass

At the top of the page we spoke of the importance of maintaining muscle while losing fat. Now that you know what percentage of your body weight is fat, it is easy to calculate how much of your weight is fat. The remainder of your body weight, which includes your muscle, bones and organs, is called lean body mass.

To calculate your body weight in terms of fat and lean body mass, first multiply your total body weight by your body fat percentage. This will give you the weight of your body fat. Now subtract this number from your total body weight, and you will have the weight of your lean body mass.

  • total body weight x body fat percentage = weight of body fat
  • total body weight - weight of body fat = weight of lean body mass

For example, if your total body weight is 200 pounds, and your body fat percentage is 25%:

  • 200 x .25 = 50 lb of body fat
  • 200 - 50 = 150 lb of lean body mass

You can now track the changes in both your body fat and lean body mass, the latter indicating whether or not there is a change in the amount of muscle on your body.

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