The feeling of becoming lighter the moment you step into a pool of water is a universally shared experience. When you are submerged, your actual mass and the force of gravity acting on it do not change. Instead, the water applies an upward counter-force that dramatically reduces the sensation of your own weight, leading to what physicists call “apparent weight loss.” Understanding how much you weigh in water is a question of fluid mechanics, volume, and density.
Buoyancy: The Upward Push
The force that opposes your weight in water is called buoyancy. This upward force results from the pressure exerted by the fluid. Water pressure increases proportionally with depth due to the increasing weight of the water column above any point. When submerged, the water pressure pushing upward on the bottom surfaces of your body is greater than the pressure pushing downward on the top surfaces. This pressure difference creates a net upward force, which cancels out a portion of your actual weight, making you feel lighter.
The Rule of Displacement
The magnitude of the buoyant force is quantified by Archimedes’ Principle. This principle states that the upward buoyant force acting on a submerged object is exactly equal to the weight of the fluid the object displaces. The weight of that displaced water is the exact measure of the upward force pushing on you. For instance, if your submerged body displaces 50 kilograms of water, the buoyant force is 50 kilograms of force, reducing your apparent weight by that amount. This relationship means that the volume of an object, not its mass, is the primary factor determining the buoyant force. If an object has a large volume but a small mass, like an inflated beach ball, it displaces a great deal of water relative to its weight, causing it to float high on the surface.
Calculating Your Apparent Weight
Your apparent weight in water is simply your actual weight minus the buoyant force. To calculate this, one must know the volume of the submerged body and the density of the water. Pure water has a density of about 1.0 gram per cubic centimeter (g/cm³), and the average human body density is very close to this value, ranging from 0.97 to 1.088 g/cm³.
Factors Affecting Human Density
Variability in human density is due to body composition and lung air volume. Fat tissue is less dense than water, while lean tissues like muscle and bone are denser. The air held in the lungs provides a large volume with almost no mass, significantly lowering the body’s overall average density. A person who takes a deep breath can easily float because their overall density falls below that of water. When fully submerged, a typical human experiences a weight reduction of 80 to 95 percent of their actual weight, depending on their density. For example, a person weighing 150 pounds on land might only have an apparent weight of 7.5 to 30 pounds when standing in water up to their neck.
Real-World Application: Hydrostatic Weighing
The principle of apparent weight loss forms the foundation of hydrostatic weighing, also known as underwater weighing. This highly accurate scientific technique determines a person’s body composition, specifically the percentage of body fat versus lean mass. The test measures an individual’s weight on land and their apparent weight when fully submerged. Using this weight difference, researchers calculate the body’s volume and total density. Since the densities of fat mass and fat-free mass (muscle, bone) are known, the overall body density measurement converts directly into a body fat percentage. A person with a higher percentage of body fat will be more buoyant and have a lower apparent weight underwater because fat tissue is less dense than lean tissue. Conversely, someone with more muscle and bone mass will be less buoyant and weigh more underwater, indicating a lower body fat percentage.