Why Do You Float in Water?

One of the very first things every learn-to-swim student practices is basic floating. Learning to find that comfortable, mostly weightless place where your legs or body don’t sink but you don’t have to exert much effort to stay stable—that’s the basis of every swimming stroke and the best place to start when you’re learning to become water safe.

But let’s back up to a fundamental question: Why do humans float in the first place?

It comes down to a complicated dance of physical forces, namely buoyancy, gravity, displacement, and density.

Buoyancy, also sometimes called upthrust, is the upward force exerted by a fluid on an object that is partially or fully immersed in it. It opposes the force of gravity, which would otherwise drag that object to the bottom of the container. Buoyancy is the reason you feel lighter or weightless when you submerge in water and mastering it means you can swim faster with less effort; the most efficient swimmers tend to ride very high in the water.

But displacement and density also factor in to how and why humans float. Displacement occurs when an object is submerged in water—the object pushes water out of the way to make room for itself. This concept was famously illustrated by ancient Greek mathematician Archimedes and his legendary “Eureka!” moment when he noted that the water level rose in the bathtub when he stepped into it.

Objects float in water if they’re less dense than the water they displace. Density is a measure of how much mass is packed into a given volume. If an object is less dense than water, it will float because the buoyant force exceeds the gravitational force pulling it downward. Turns out, the human body, which is about 60% water, floats because it is slightly less dense than water. Each body part has its own density level; the lungs are less dense than solid muscle, for example.

It’s true the ability to float well can vary from person to person. It’s also true that adipose tissue is less dense than muscle, and these reserves, marbled throughout the body, help keep your body at the surface. But the volume of air in the lungs and other factors can impact the ease with which you float.

While it’s certainly possible for everyone to learn how to achieve the right equilibrium on the surface of the water to achieve a nice, streamlined floating position, that skill may come more naturally to some people than others. This has mostly to do with body composition—the ratio of fat to denser tissue. But your body position and where your center of gravity is at a given moment can also greatly influence your ability to float. For example, if you ball up tight, it’s much harder to stay at the surface than if you lie flat with arms and legs extended. That’s because by spreading out, you increase the surface area against with buoyancy can exert its force to help overcome gravity’s pull.

For all swimmers, being able to float comfortably is a key baseline component to improving your stroke. If you’re someone who often struggles to find your balance in the water, take a moment to practice it or reach out to a coach for assistance. Finding that sweet spot where you can lie effortlessly on the water’s surface will help you become a better swimmer in the long run.