The density of muscle is about 18 percent greater than that of fat. So while a pound of fat and a pound of muscle weigh the same amount, five pounds of muscle occupy approximately the same amount of space as four pounds of fat.
The difference between the density of fat and muscle means that a cubic inch of muscle weighs more than a cubic inch of fat. For people who are working out to lose weight, this often leads to frustration because their weight remains constant after they drop some initial water weight. Looking at one's body fat percentage or waist measurement is a better way to gauge progress when exercising than relying on the scale alone.