Why do old eggs float while fresh ones sink?

A large chicken egg is 6 cm (2.4 in) tall, yet it can sink when fresh and float later. As it ages, moisture escapes through thousands of microscopic pores in its shell, while the air cell at its blunt end expands and the egg loses mass. Its outside changes little, but the balance between its mass and volume shifts.

The egg keeps its outline as its contents change

A chicken egg is about ten grains of rice long. Density means how much mass occupies a given volume. An egg’s shell keeps roughly the same outer dimensions as its contents change, so its size alone cannot explain why it rises or sinks. The important change happens inside the shell.

The water test makes that hidden change visible. It compares the weight of the whole egg with the water it pushes aside. An egg can keep its familiar shape while losing enough mass to behave differently in a bowl.

What escapes through eggshell pores

An eggshell is made mostly of calcium carbonate and has thousands of microscopic pores that allow gas exchange. The pores are part of the shell, not cracks or damage. They let moisture and gases pass between the egg and the surrounding air while the shell remains intact.

Water vapour and carbon dioxide leave through those pores as the egg ages. The University of Georgia explains (opens in a new tab) how gases and moisture pass through eggshell pores, changing the space inside over time. With moisture leaving, the egg loses mass; the shell’s outside does not need to grow or break for that change to happen.

That slow exchange is why looking at an uncracked shell tells you little about the egg’s age. The shell can still look smooth while the contents gradually lose moisture. The change is internal, and the growing air cell is where part of it becomes visible.

Where the air cell grows

At the egg’s blunt, wider end, the shell membranes separate to form an air cell. This is a pocket between the membranes, not a bubble loose in the egg white. As moisture leaves and the contents contract, the pocket expands to occupy more of the space inside the shell.

The growing air cell is not an air balloon pushing the shell outward. The shell keeps its outline while the contents shrink and the pocket at the blunt end gets larger. That shift, alongside the egg’s loss of mass, changes the egg’s average density.

A bright light held against an intact egg can help reveal the air cell through the shell. This method, called candling, lets you see the pocket’s boundary without opening the egg. The University of Wisconsin–Madison Extension guide (opens in a new tab) explains that the float test reflects the air cell’s size and that storage conditions change how quickly the air cell grows.

Why a fresh egg sinks and an older one floats

In plain water, a fresh egg sinks because its average density is greater than the water’s. Gravity pulls the egg downward, while the water pushes upward on the egg according to how much water it displaces. If the egg is denser than the water it pushes aside, it sinks.

As moisture escapes, the egg’s mass falls while its outer volume stays nearly the same. Its average density drops. When the water it displaces can support the egg’s weight, the egg floats. The expanding air cell is part of the visible change, but the key is the balance between the egg’s total mass and its volume.

To try the test, lower an intact egg into a bowl of plain water and watch where it settles. Salt water is denser than plain water, so it can support an egg more easily. The result therefore depends on the water as well as on the egg; plain water gives the clearest everyday reading of the change inside the shell.

What the float test can and cannot tell you

An egg that floats has an enlarged air cell and has lost more moisture than a fresh egg. The USDA’s Shell Eggs from Farm to Table (opens in a new tab) explains that an egg floats when its air cell is large enough to make it buoyant. The test signals a change associated with age, but it cannot tell you whether harmful bacteria are present.

Age and safety are different questions. The float test reports how the egg behaves in water; it does not replace safe storage and handling guidance. For eggs, use food-safety advice rather than the egg’s position in a bowl to decide whether it is safe to eat.

The float test tells you what changed inside the shell, not whether the egg belongs in the pan.