A good friend of mine threw out the bathroom scale one morning because his wife was too depressed to make love after she weighed herself and discovered she'd gained two pounds.

Many weight-conscious Americans let the scale rule their mood, which is one reason why some fitness professionals contend that trashing the scale can be a pretty good idea. But another important argument for scale-free living is that, since we all have unique builds and genes and fitness levels, scale weight can be deceiving.

For example, a six-foot-tall, sedentary man who weighs 185 pounds may actually be fatter than a six-foot-tall Redskin halfback who weighs 220. Two people of the same height and weight can have vastly different shapes -- and health risks -- if one person's weight is mostly muscle and bone, and the other person's is excessively fat.

"The scale provides a meaningless number because it doesn't indicate the composition of the pounds," writes Massachusetts nutritionist Nancy Clark in her new "Sports Nutrition Guidebook" (Leisure Press, 1990). "Some pounds are generally desirable muscle weight, others are less desirable fat weight."

So it's not how much you weigh that matters to your health -- it's how fat you are. If you rely solely on the scale to judge your fitness, you could be badly misled.

New exercisers can get a false sense of despair from the scale, since exercise can build muscle, and muscle has weight. After six weeks of working out, a person may lose fat and drop a clothing size, but the scale may not indicate much change. Likewise, the scale can give a false sense of security to sedentary people who keep their weight constant but are actually getting fatter.

"Even if you weigh the same as you did 20 years ago, you probably are fatter than you were then," says exercise physiologist Wayne Westcott, strength-training consultant to the YMCA of the USA.

"After age 20, adults who do not perform some type of strength training lose about one pound of muscle every two years. So a (sedentary) 40-year-old woman who weighs the same as she did in college has simply replaced 10 pounds of muscle with 10 pounds of fat. Although her body weight is the same, her body composition has changed markedly." This is why measuring body fat composition has become increasingly important to professional athletes and fitness-minded Americans who want to know what proportion of their body is muscle and bone and what part is fat. While some fat is essential for the body to function properly, most Americans carry around excess fat.

How much fat is too much? Opinions vary, but the generally accepted normal percentage of fat for males ranges from about 10 to 17 percent of total body weight. For females, who have more fatty tissue than men, the normal range is about 17 to 26 percent.

Athletes typically desire lower proportions of body fat, since excess fat can slow them down. The Cooper Clinic in Dallas sets a maximum athletic body-fat weight for men at 15 percent and for women at 18 percent. "We see lots of athletes in the 4 to 10 percent body-fat range," says Kenneth Cooper, a physician and chairman of the clinic. "And we've tested men with as little as 3 percent body fat."

But it's important not to drop fat levels too low, since excessively low body fat is linked to some abnormalities, such as menstrual irregularities in women.

Three common ways to estimate percent body fat are:

1. Underwater (hydrostatic) weighing, which relies on Archimedes's principle of water displacement to calculate body density and determine percentage fat. In this procedure, you sit in a chair in a vat of water, exhale all the air in your lungs and stay submerged for about 10 seconds without moving.

While hydrostatic weighing is considered the "gold standard" for accuracy among many professional athletes, errors can occur. If you don't exhale all the air in your lungs or if you have intestinal gas, readings may be significantly affected.

2. Electrical impedance, which is a computerized system that sends an imperceptible electric current through your body by way of an electrode attached to your wrist or ankle. The amount of water in your body affects the current flow. Since water is only in fat-free tissue, the computer can calculate your percentage of body fat. But errors can occur if something has affected your body's water level, such as hard exercise, dehydration, drinking or premenstrual bloat.

3. Skin-fold calipers, the most commonplace method, are large pincers that measure the thickness of your fat layer at three to six specific body sites. Inaccuracies here are often the work of poorly trained technicians at crowded health fairs -- but they can also be the result of individual fat patterns.

Since all three methods have a strong potential for inaccuracy, Clark says, "consider body-fat measurement as a measurement against itself that reflects changes in your body as you lose fat, gain muscle, shape up and slim down. The standard error is plus or minus 3 percent.

"Your best bet is to see how the measurements change over time. Have the same person take them at bimonthly intervals over the course of a year. Calipers are generally the most convenient, most precise and least expensive method."

While knowing your body-fat percentage can be a useful tool, probably the best way to judge your fatness is to take a realistic look at your body.

Try this low-tech, "see fat" test: Take your clothes off in front of a mirror. Move around and see if anything jiggles that nature didn't intend. Pinch yourself at trouble spots, and see if you can lift skin folds thicker than your index finger. Chances are, if you look overfat, you probably are.

Next time: Working off body fat. Bodyworks appears on alternate Tuesdays.