You might assume that fat people would sink in water, right?
After all, fat people do typically weigh more and heavier things sink. But fat folks are actually excellent at floating!
So why do fat people seem to float so well? It has to do with density. Fat is actually less dense than water which means it floats. If you add oil to a pot of water, you’ll see it floating on top and it’s the same idea when a fat person gets in the water. Muscle is denser than water and so it sinks.
Let’s take a closer look at what’s going on here and what it means for fat folks.
What Determines Who Floats…And Who Doesn’t?
The physics behind floating in water is a lot more complex than just weight. Instead, it all comes down to density and displacement.
Understanding Density, Specific Gravity, and Floating
Don’t be intimated by scientific jargon like specific gravity! We’re going to keep this as simple as possible but we do need to dive into a little science to explain why fat folks are more likely to float.
The first concept we need to understand is density.
Density describes the mass per unit of volume for a particular substance or in simple terms how tightly “packed” something is. You can also think of density as a way of measuring how heavy something is compared to its overall size.
For a good example of this, consider two balls of the same size. One is hollow and made from plastic and another made of lead. You know, without even thinking, that the hollow plastic ball will not only weigh less but will also float on water while the lead ball will sink. The balls are the same size but their main difference comes down to density.
It’s easy to imagine the density of a solid object, but liquids like water have a density too and objects that are less dense than water will float while objects that are denser than water will sink.
When people describe the density relationship between two substances they sometimes just the term specific gravity which describes that ratio with a single number. While it depends on the temperature, most folks will describe the specific gravity of water as 1.0.
That means anything with a specific gravity greater than 1.0 will sink (because it’s denser than water) while anything with a specific gravity less than 1.0 will float.
The Density of Fat vs Muscle
So what does all this have to do with fat people floating?
By understanding the density of fat, muscle, and water we can fully understand why some folks float and others don’t!
Fat has a density of 0.901 g/cm3, muscle has a density of around 1.1 g/cm3, and water has a density of 1.0 g/cm3. In other words, fat is less dense than water and so it’s more buoyant while lean tissues like a muscle are denser than water so it tends to sink.
This also explains why people with a lot of muscle tissue weigh more than you’d expect just by looking at them. All that muscle is dense (and heavy)! Too heavy to float in water!
This video has a great little experiment that illustrates how the density of a liquid can interact with a solid and will help you understand these ideas:
What About Other Tissues?
What’s About Density and Buoyancy?
When you get into a bathtub, what happens?
Well, if you’re me then most of the water spills out because there just isn’t enough room for me and the water. That’s called displacement and it’s what happens when anyone enters any body of water but it’s most noticeable in something smaller like a bathtub. If you’re bigger, like me, then you’re displacing even more water.
But it doesn’t end there.
When you enter the bathtub and displace water, you’re also going to experience an upward force from the water. That force is called buoyancy and it’s a word you’ve probably heard before.
That force of the water depends on the amount of water displaced and you can see a great example of this here. Fat people will often displace more water which means they create a greater buoyancy force that helps them float.
Total Body Composition Is What Matters
With all the different tissues that make up your body and their different densities, it should be clear that the total composition of a body is what determines if you’re more likely to float or not.
For example, a person with a ton of muscle would typically sink but if that muscle is covered with fat, enough to change their average density, then they could float!
Of course, there are even more variables to consider like the temperature of the water, whether it’s salt or freshwater, and what you’re doing to try and increase your buoyancy.
But in general, fat people are more likely to float in water because the density of fat is less than water. On the other side of things, because lean tissue has more density than water, muscular people like bodybuilders are more likely to sink.
People (Including Fat Ones) Float Better In Cold Water
Everyone floats better in cold water. That’s because cold water is denser than warm or room temperature water. Remember, we just explained that floating is all about the relationship between the density of the object and the density of water. So if you can increase the density of water, by decreasing the temperature, not only will fat people float but suddenly some other folks will too.
The idea of water increasing in density as it gets colder makes sense at a molecular level but it also checks out when you consider what happens to water when it gets really cold…you know, like ice! Ice is obviously more dense than water, which is also why ice cubes float in liquids.
That cube is like a densely packed muscle!
The opposite is also true and warmer water is less loosely packed and so it’s got less density.
In most cases, you won’t notice much of a difference unless you’re dealing with some rather extreme temperatures.
Salt Water Vs Fresh Water
Saltwater is denser than fresh water and as result, everyone, including fat folks, is more likely to float in it. If your body composition is average or even a little muscular, you might find that you sink in freshwater but float a little better in the ocean.
That’s because salt water has a density of around 1.02 g/cm3 to 1.03 g/cm3 which makes it just a little denser than your standard water.
But then there are more extreme examples like the Dead Sea which has a density of 1.24 g/cm3!
But just how big a difference is that compared to the density of other water?
Check out this bowling ball, along with a handful of pins, floating in the Dead Sea to really see the power of density!
What About Pools?
Chlorine is denser than water with a density of 1.56 g/cm3 however there’s a minimal amount of chlorine in the water so the impact on the overall density is pretty minimal. As a result, you’re not likely to notice much of a difference, if any, compared to fresh water and the pool.