Why Does An Egg Float In Water? The Reason

An egg floats in water because it has lost moisture through its porous shell, leaving a larger air cell inside. That extra air makes the egg less dense than the water around it, so it rises to the surface. The same principle explains why a fresh egg lies flat on the bottom of a glass. Floating is not a random quirk. It is a direct measure of how much air has replaced water inside the shell.

Why Does An Egg Float In Water?

An egg is not a sealed container. Its shell is covered in thousands of tiny pores that let gases and moisture pass through over time.

When an egg is laid, the shell has two membranes pressed tightly together at the wide end. As the egg ages, water inside the white evaporates through the pores. Air from outside moves in to take its place. That air collects in a pocket called the air cell, which sits at the wide end of the egg.

A fresh egg has a small air cell. A weeks-old egg has a much bigger one. Once the air cell grows large enough, the egg’s total density drops below that of water. Density is simply how much mass is packed into a given space. Water has a density of about 1 gram per milliliter. A fresh egg is slightly denser than water, so it sinks. An old egg, with a big air pocket, is slightly less dense, so it floats.

This is the same physics that keeps a hollow ball on the surface of a pool. The ball is not lighter than the water by weight alone. It is lighter for its size because so much of its volume is air.

What The Float Test Actually Tells You

The float test is a rough freshness gauge, not a safety test. Those are two different questions.

When you lower an egg into a bowl of water, its position gives you a general sense of age:

• Lies flat on the bottom: very fresh
• Sits upright on the bottom but does not float: a few weeks old
• Hovers just below the surface: older, but the air cell is not yet large enough to lift it
• Floats at the surface: old, with a large air cell

Here is the part most people get wrong. Floating tells you the egg has lost water and gained air. It does not tell you whether bacteria have entered the shell. A floating egg may still be perfectly safe to eat. A sinking egg can still be contaminated. The float test measures age and air content. It does not measure bacterial load.

The USDA and other food safety agencies are clear on one point. An egg that floats should be discarded, not because floating itself makes it unsafe, but because an egg old enough to float is old enough that spoilage becomes a real possibility. The float test is a useful first screen. It is not the final word.

How The Air Cell Forms And Grows

The air cell is not present in a freshly laid egg in any meaningful size. It begins to form as soon as the egg cools after laying.

An egg leaves the hen at roughly the hen’s body temperature, around 105 degrees Fahrenheit. As it cools to room temperature, the contents shrink slightly. That shrinkage pulls the inner shell membrane away from the outer one at the wide end, creating a small gap. Air seeps through the shell pores and fills that gap. This is the beginning of the air cell.

From that point on, the air cell grows slowly. How fast depends on several factors:

• Temperature. Warm storage speeds up moisture loss. Refrigeration slows it down.
• Humidity. Dry air pulls moisture out of the egg faster than humid air does.
• Shell quality. Thinner or more porous shells lose moisture more quickly.
• Time. The longer an egg sits, the larger the air cell becomes.

This is why commercial egg cartons carry a pack date and often a use-by date. Those dates are based on how quickly the air cell and the egg’s internal chemistry change under normal storage. They are not arbitrary.

Does A Floating Egg Mean It Is Bad?

Not necessarily. Floating means the egg is old. Old and spoiled are not the same thing.

An egg can be several weeks old, float in water, and still be safe to eat if it has been stored properly and never cracked. Egg spoilage is usually caused by bacteria, most commonly Salmonella, entering through the shell or being present before the shell forms. Proper refrigeration and handling are what keep that risk low, not the float test.

A better way to check an egg is to crack it open and use your senses:

• A spoiled egg usually has a distinct, unpleasant smell. Trust your nose.
• The white of an old egg is thinner and spreads out more. A fresh egg’s white stays thick and holds its shape.
• The yolk of an old egg is flatter and more prone to breaking.
• Any egg with a cracked shell, a slimy texture, or an off odor should be thrown away.

If an egg floats and you crack it and it smells fine, the decision is still yours. Many food safety sources recommend discarding floating eggs as a precaution. That is reasonable, especially if you are cooking for anyone in a higher-risk group, such as young children, older adults, or people with weakened immune systems.

Why Fresh Eggs Sink And Old Eggs Float

The difference comes down to what is inside the shell, not the shell itself.

A fresh egg is mostly water. Its white and yolk are dense, and the air cell is tiny. The overall density is greater than water, so it sinks and settles on the bottom of the glass. As the egg ages, water leaves and air enters. Air is far less dense than water. Once enough water has been replaced by air, the egg’s average density falls below that of water, and the egg floats.

You can think of it as a slow swap. Water out, air in. The shell stays the same size the whole time. What changes is the contents.

This is also why a very old egg feels lighter in your hand than a fresh one, even though the shell looks identical. You are holding less water and more empty space.

How To Store Eggs So They Stay Fresh Longer

Refrigeration is the single biggest factor in slowing the air cell’s growth and keeping an egg fresh.

In the United States, eggs are typically washed and refrigerated after collection. Washing removes a natural protective coating called the bloom, which is why US eggs are refrigerated from the store to your kitchen. In many other countries, eggs are not washed and are often stored at room temperature. Both approaches are used, and each reflects a different set of food safety practices. If you buy refrigerated eggs, keep them refrigerated.

A few practical points:

• Store eggs in their original carton inside the fridge, not in the door. The door is the warmest part and the temperature swings every time you open it.
• Keep eggs at a steady cold temperature. Consistency matters as much as the number on the dial.
• Use eggs within the timeframe on the carton. The pack date and use-by date are reliable guides.
• Do not wash eggs before storing them. Moisture on the shell can help bacteria move through the pores.

None of this is complicated. Cold, steady, and unwashed until you are ready to cook is the general rule for refrigerated eggs.

What The Float Test Cannot Do

The float test is a simple household tool, and it has real limits. It cannot detect Salmonella or other bacteria. It cannot tell you whether an egg was stored properly. It cannot tell you the exact age of an egg, only whether it has lost enough moisture to change its buoyancy.

Some people treat the float test as a definitive safety check. It is not. It is one piece of information among several. A smell test, a visual check of the cracked egg, and attention to storage conditions all matter more for safety than whether the egg bobs in a glass of water.

If you are ever unsure about an egg, the safe choice is to throw it out. The cost of one egg is small compared with the risk of a foodborne illness.