An egg that floats in a glass of water has lost enough moisture and carbon dioxide through its shell that the air pocket inside has grown large enough to make the egg buoyant. A fresh egg has a density of roughly 1.03 grams per milliliter, which is just above that of plain water, so it sinks. As days and weeks pass, that density drops below 1.0, and the egg rises. Floating signals that an egg is old, but old and unsafe are not the same thing, and the distinction matters more than most kitchen advice lets on.
Why Eggs Sink or Float
Every egg contains a small air pocket, called the air cell, nestled between the shell membrane and the shell at the broad end. When an egg is freshly laid, this pocket is tiny. Over time, moisture and carbon dioxide escape through thousands of microscopic pores in the shell, and outside air seeps in to replace them. The air cell grows. Because air is far less dense than egg white or yolk, a bigger air cell lowers the egg’s overall density. Once that density drops below 1.0 g/mL, the egg becomes buoyant in plain water.
A fluid-dynamics experiment using plain water, saltwater, and a baking-soda solution demonstrated this neatly. In plain water (density of 1,000 kg/m³), the egg sank. In saltwater at 1,020 kg/m³, it partially floated. And in a baking-soda mixture at 1,100 kg/m³, it bobbed well above the bottom, because the denser fluid generated a much stronger upward push.1Semarak Journal of Thermal Fluid Engineering. An Analysis of Egg Floating and Sinking in Fluids of Three Different Densities This is worth knowing because it means the test only works in plain, unsalted tap water. Drop an egg into broth or salted water and you’ll get a misleading result.
The Four Stages You Can Actually See
The water test is more informative than a simple sink-or-float binary. A physics analysis of egg orientation in water identified four distinct positions that correspond to increasing age. A very fresh egg sinks and lies flat on its side at the bottom of the glass, because its center of mass is evenly distributed and the air cell is negligible. After roughly a week, the air cell has grown enough to tip the broad end slightly upward, so the egg sinks but rests at an angle with the pointed end still touching the bottom. A few days later, the egg stands upright on its narrow tip, broad end pointing straight up. Finally, when enough air has accumulated, the egg lifts off the bottom entirely and floats just beneath or at the surface.2IOP Publishing. Floating eggs?—the answer
These positions are useful because they give you a rough timeline, not just a yes-or-no verdict. An egg standing on its tip is older than one lying flat but still has weeks of usable life ahead of it under proper refrigeration. Only the fully floating egg should prompt closer inspection before you crack it open.
What Actually Changes Inside an Aging Egg
The air cell growth that makes an egg float is just one visible sign of a cascade of chemical changes. The most important one for both safety and cooking is the rise in pH. A freshly laid egg’s albumen (the white) has a pH around 7.6 to 7.9. Within the first three days at room temperature, the pH climbs sharply, and it levels off after about a week in the range of 9.0 to 9.4.3Poultry Science. Chemical and Related Osmotic Changes in Egg Albumen During Storage This happens because dissolved carbon dioxide escapes through the shell, and the albumen becomes more alkaline as a result. The thick, gel-like white that holds a fresh yolk in place gradually thins out and becomes watery.
Temperature is the main accelerator of all these changes. Research on egg quality found that the air cell can exceed 4 mm in depth after just two days when eggs are stored above 21°C (about 70°F).4Journal of Applied Poultry Research. Effects of Storage Time and Temperature on Egg Quality in Old Laying Hens Refrigeration slows moisture loss and CO₂ escape dramatically. Kinetic modeling of egg freshness confirmed that temperature is the single strongest predictor of how quickly quality declines, with relative humidity playing a secondary role and airflow having a negligible effect on weight loss.5PubMed Central. Kinetic modeling impacts of relative humidity, storage temperature, and air flow velocity on various indices of hen egg freshness In practical terms, eggs left on a counter age in days what refrigerated eggs take weeks to reach.
Does Floating Mean Unsafe?
Here is where the common advice gets sloppy. Many sources treat the float test as a safety test. It is not. The float test is a freshness test. It tells you the air cell has expanded significantly, which means the egg is old. But the primary safety concern with eggs is Salmonella Enteritidis, and Salmonella contamination has nothing to do with how big the air cell is.
Salmonella can be deposited inside the egg before the shell even forms, through infection of the hen’s reproductive tract. It can also reach the interior after laying if bacteria on the shell surface penetrate inward. A risk assessment of U.S. shell eggs estimated that roughly 2.3 million Salmonella-contaminated eggs are produced out of about 69 billion laid annually.6PubMed. An overview of the Salmonella enteritidis risk assessment for shell eggs and egg products That is a tiny fraction, but because eggs are consumed in such enormous quantities, the total number of resulting illnesses each year is large. Salmonella Enteritidis remains one of the most common foodborne pathogens linked to shell eggs.7Comprehensive Reviews in Food Science and Food Safety. Control of Salmonella Contamination of Shell Eggs—Preharvest and Postharvest Methods: A Review
The key point is that a perfectly fresh egg that sinks straight to the bottom can carry Salmonella, while a floating egg that is simply old may be completely free of pathogens. The float test catches staleness, not contamination. Proper cooking to at least 160°F (71°C) throughout the egg kills Salmonella regardless of the egg’s age. Refrigeration slows bacterial multiplication but does not eliminate bacteria that are already present.
When a Floating Egg Is Genuinely Spoiled
So how do you know if a floating egg has crossed the line from “old but usable” to “throw it away”? Your nose is a better judge than the water glass. Bacterial spoilage produces unmistakable odors. Certain bacteria that break down egg whites generate a fishy smell, while Pseudomonas species, which are common spoilage organisms, produce a strong odor resembling cabbage water.8Epidemiology & Infection. Observations on the bacterial flora of the hen’s egg, with a description of new species of Proteus and Pseudomonas causing rots in eggs If you crack a floating egg into a bowl and smell nothing unusual, the egg is almost certainly safe to cook and eat. The white will be thin and runny, and the yolk may be flat rather than domed, but these are quality issues, not safety hazards.
Visual signs of spoilage include unusual discoloration, such as pink, green, or iridescent patches in the white, and an off-putting slimy texture. A yolk that breaks the moment it leaves the shell simply has a weakened membrane from age. Unpleasant as the texture may be, a weak yolk membrane by itself does not indicate bacterial contamination.
The Shell’s Built-In Defense System
Understanding why older eggs become more vulnerable to bacteria helps explain when the float test’s freshness information does carry safety implications. The outermost layer of an eggshell is a thin, waxy coating called the cuticle. It plugs the shell’s respiratory pores, creating both a physical and chemical barrier against microbes trying to get in.9Frontiers in Immunology. Properties, Genetics and Innate Immune Function of the Cuticle in Egg-Laying Species The cuticle is deposited during the last couple of hours of shell formation inside the hen and is composed mainly of glycoproteins. It is not calcified like the shell itself, which makes it somewhat fragile.
Bacterial contamination progresses through stages: bacteria first attach to the shell surface, then penetrate the cuticle and shell, multiply in the underlying membranes, and finally reach the egg’s contents.10PubMed Central. Effect of Egg Washing and Hen Age on Cuticle Quality and Bacterial Adherence in Table Eggs As the cuticle degrades over time, or when it is stripped away by washing, the pores become exposed. That is why eggs sold in the United States are required to be washed and then refrigerated: washing removes the cuticle along with surface bacteria, so refrigeration becomes the primary barrier against new contamination. In many European countries, eggs are not washed commercially and can be stored at room temperature because the cuticle remains intact. Both systems work, but you cannot mix and match. A washed egg left unrefrigerated loses its main defense.
The practical implication for the float test: a very old egg has had more time for its cuticle (if it still had one) to degrade and for any surface bacteria to work their way inward. So while floating does not equal contaminated, a floating egg is statistically more likely to have been penetrated by spoilage organisms than a fresh one, especially if it has been stored poorly.
Storage Temperature Matters More Than the Date Stamp
The “sell by” or “best by” date on a carton is a rough guide to freshness, but it tells you nothing about how the eggs were actually handled. An egg that was accidentally left out on a loading dock for a few hours on a hot day ages faster than one that stayed cold the entire time. Research consistently shows that temperature is the dominant variable in egg quality decline.5PubMed Central. Kinetic modeling impacts of relative humidity, storage temperature, and air flow velocity on various indices of hen egg freshness Two eggs with the same lay date can be in completely different states depending on cold-chain handling.
This is why the float test actually gives you more practical information than the printed date in many cases. It reflects the cumulative effect of whatever temperature history the egg has experienced, regardless of what the carton says. If you have eggs that are a few days past their sell-by date and they sink and lie flat, they are likely in better shape than eggs within their sell-by window that were stored carelessly.
Older Eggs Are Actually Better for Some Recipes
A detail that rarely makes it into the “should I throw this away” conversation is that older eggs perform better for certain cooking tasks. The most well-known example is hard-boiled eggs. Fresh eggs are notoriously difficult to peel cleanly because the albumen clings to the inner membrane. Research on peelability found that the critical variable is egg-white pH. Below roughly 8.7 to 8.9, peeling tends to be messy and frustrating, while above that range, the shell separates cleanly.11Elsevier / ScienceDirect. Peelability of Hard-cooked Eggs Since a fresh egg’s white sits at a pH well below that threshold and rises past it within days, eggs that have been in the fridge for a week or two are ideal candidates for boiling.
Eggs standing on their tips in the water test, the stage-three position, hit a sweet spot for this purpose. They are old enough that the pH has climbed well past the peelability threshold, but fresh enough that the yolk is still centered and the white still has some structure. For meringues and other whipped-egg applications, though, fresh eggs are better. The higher viscosity and lower pH of a fresh white produces a more stable foam.
Professional Freshness Tests Beyond the Kitchen
The float test is a home cook’s tool. In commercial egg grading and food-science research, freshness is measured more precisely using a metric called the Haugh unit, which is based on the height of the thick albumen when an egg is cracked onto a flat surface. A tall, firm mound of white earns a high Haugh unit score; a flat, watery spread earns a low one. Candling, which involves shining a bright light through the shell to view the air cell and yolk shadow, is the other traditional commercial method.12PubMed. Egg Freshness Safety and Detection Techniques: A Comprehensive Review and Future Perspective Both have been standard in the industry for decades, though the Haugh unit test requires cracking the egg open and candling has limits with very thick or pigmented shells.13Agriculture. Non-Destructive Evaluation of Physicochemical Properties for Egg Freshness: A Review
These tests confirm the same general story the float test tells: as eggs age, the white thins, the air cell grows, and measurable quality drops. The float test just happens to be a zero-cost, non-destructive version you can do with a glass of tap water.
Edible Coatings and Shelf-Life Extension
If the float test essentially tracks how fast the shell loses moisture and CO₂, it follows that anything slowing that loss would keep eggs sinking longer. Researchers have tested food-grade coatings that seal the shell pores. Eggs coated with paraffin wax, mineral oil, soy protein, or whey protein and stored at refrigerator temperatures maintained higher Haugh units for over six weeks compared to uncoated eggs. Coated eggs also held onto more of their CO₂ and had lower albumen pH than uncoated ones through the same storage period.14PubMed. The effects of edible coatings on chicken egg quality under refrigerated storage A more recent study found that shellac-based nanocomposite coatings sealed shell pores effectively, reduced gas diffusion, and improved both shell strength and storage stability.15International Journal of Biological Macromolecules. The shellac and shellac nanocomposite coatings on enhanced the storage stability of fresh eggs for sustainable packaging
You will not find shellac-coated eggs at most grocery stores yet, but mineral-oil treatments are already used in some commercial settings, and the principle is the same one behind the old farmhouse trick of coating eggs in lard or petroleum jelly for long-term pantry storage. These methods effectively replace the natural cuticle that commercial washing strips away, buying time before the air cell expands enough to make the egg float.
Backyard and Farm Eggs Behave Differently
If you keep chickens or buy eggs from a local farm, the float test still works the same way, but the timeline shifts. Unwashed eggs with an intact cuticle lose moisture much more slowly than washed, refrigerated supermarket eggs because those sealed pores restrict gas exchange. An unwashed farm egg stored at cool room temperature may take considerably longer to progress through the four float-test stages than a commercially processed egg stored in the fridge, even though both are losing freshness through the same basic mechanism.
The flip side is that unwashed eggs can carry surface bacteria, including Salmonella, on the shell. While the intact cuticle prevents most of those organisms from penetrating inward, handling unwashed eggs and then touching other food can lead to cross-contamination. If you choose to wash farm eggs before storing them, you have effectively moved them into the same category as supermarket eggs: refrigerate them promptly and expect the same rate of freshness decline.
One more wrinkle worth knowing: hen age affects cuticle quality. Older hens tend to deposit a thinner, less complete cuticle, which means their eggs may lose freshness faster and be more susceptible to bacterial penetration than eggs from younger birds, even under identical storage conditions.10PubMed Central. Effect of Egg Washing and Hen Age on Cuticle Quality and Bacterial Adherence in Table Eggs If you notice that eggs from your older hens seem to go off more quickly, this is likely the reason.