The quickest way to check whether an egg has gone bad is to drop it into a bowl of cold water. A fresh egg sinks and lies flat on the bottom. An egg that tilts upward or floats has lost enough moisture through its shell to signal it’s past its prime, and one that bobs at the surface is almost certainly spoiled. But the float test is just one of several reliable checks you can do at home, and combining a few of them gives you the most confident answer.
The Float Test and Why It Works
Fill a bowl or tall glass with enough cold water to cover an egg by a couple of inches, then gently lower the egg in. What you’re really measuring is density. Eggshells are covered in thousands of tiny pores, and from the moment an egg is laid, moisture and carbon dioxide slowly escape through them. That loss shrinks the egg’s internal mass and causes the air pocket at the wide end to grow larger. A fresh egg is denser than water, so it sinks. As days and weeks pass, the expanding air cell makes the egg increasingly buoyant.
Here’s how to read the results:
- Sinks and lies flat on its side: very fresh, likely less than a week or two old.
- Sinks but stands upright on one end: older but still safe to eat. The air cell has grown, but the egg hasn’t spoiled. These are actually ideal for hard boiling because the larger air pocket makes peeling easier.
- Floats to the surface: the egg has lost significant moisture and gas buildup is substantial. Discard it or crack it open somewhere you can inspect and smell it before deciding.
The float test is a good first filter, but it measures age, not contamination. An egg can sink and still harbor bacteria, or float and technically not be rotten yet. That’s why the next checks matter.
The Smell Test
Nothing beats your nose. The unmistakable rotten-egg smell comes from hydrogen sulfide, a gas produced when proteins in the egg break down. Humans can detect it at remarkably low concentrations, as little as 0.008 parts per million in air, which means if an egg is producing even trace amounts, you’ll know.
A truly spoiled egg will hit you the moment you crack the shell. If you’re unsure about an egg, crack it into a small bowl or cup rather than directly into your pan. A fresh egg has almost no odor at all, just a faint, clean, slightly sulfurous note. Anything sharp, sour, or unmistakably foul means the egg should go in the trash. This applies whether the egg is raw or cooked: a spoiled egg that somehow made it into a pot of boiling water will still smell off once you open it.
Visual Signs of Spoilage
Before you crack the egg, look at the shell. Cracks, sliminess, or a powdery or chalky texture that wasn’t there when you bought the carton can all indicate bacterial growth. A cracked shell is an open door for microbes, so even if the egg smells fine, the risk isn’t worth it.
Once cracked into a bowl, a fresh egg has a thick, slightly cloudy or clear white that holds its shape around a firm, rounded yolk. As eggs age, the white thins out and spreads across the bowl, and the yolk flattens. Those changes alone don’t mean the egg is dangerous, just older. What should concern you is color. Pink, green, or iridescent streaks in the egg white point to bacterial contamination, specifically a type of bacteria called Pseudomonas. Any unusual color in the yolk or white, or black or green spots, means the egg is spoiled. Throw it away.
A small blood spot on the yolk, by contrast, is harmless. It’s caused by a tiny blood vessel rupturing during the egg’s formation and has nothing to do with freshness or safety.
The Shake Test
Hold the egg close to your ear and give it a gentle shake. In a fresh egg, the contents are thick and tightly packed, so you won’t hear anything. In an older egg, the white has thinned out and the yolk has loosened from its anchoring membranes, so you may hear a sloshing or swishing sound. That sloshing usually points to an old, watery yolk. This test is less reliable than the float or smell test, though. A slight sound might just mean the egg is a few weeks old, not that it’s spoiled. Use it as a supporting clue rather than a final verdict, and always follow up with a visual and smell check after cracking.
Reading the Carton Date
Egg cartons carry two numbers worth knowing. The first is the pack date, printed as a three-digit code called a Julian date. It represents the day of the year the eggs were washed and packed: 001 is January 1, 032 is February 1, and 365 is December 31. If you see “045” on the carton, that means the eggs were packed on February 14. Counting forward from that date gives you a solid idea of how old the eggs actually are.
The second number is the sell-by or expiration date, which varies by state regulations. Eggs stored in the refrigerator at 40°F (4°C) or below are generally safe for three to five weeks past the pack date. So even if you’re a day or two past the printed sell-by date, the eggs may still be perfectly fine. Use the tests above to confirm rather than tossing them automatically.
What Freshness Tests Can’t Detect
Here’s the critical caveat: a perfectly fresh-looking, good-smelling egg can still make you sick. Salmonella bacteria can be present inside the egg before the shell even forms, introduced during development inside the hen. It can also enter through the shell’s pores after laying. Salmonella does not cause any visible change in the egg’s appearance, smell, or texture. You cannot detect it at home.
This is why proper storage and cooking matter as much as freshness checks. Keep eggs refrigerated from the time you buy them. Cook them until both the white and yolk are firm, which means reaching an internal temperature high enough to kill the bacteria. Runny yolks carry more risk, especially for young children, older adults, pregnant women, and anyone with a weakened immune system.
If you do eat a contaminated egg, symptoms of Salmonella poisoning typically include stomach cramps, diarrhea, nausea, and fever. They can appear anywhere from six hours to six days after eating the contaminated food and usually last four to seven days.
A Quick Decision Framework
When you’re standing in the kitchen with a questionable egg, run through these steps in order:
- Check the shell: cracks, slime, or unusual texture means discard immediately.
- Do the float test: if it floats, proceed with caution or toss it. If it sinks, move on.
- Crack it into a separate bowl: never crack a suspect egg directly into a recipe or hot pan.
- Look at the color: pink, green, or iridescent whites mean bacterial contamination. Toss it.
- Smell it: any sour or sulfurous odor beyond the faintest hint means it’s spoiled.
If the egg passes all five checks, it’s safe to cook and eat. When in doubt at any step, the cost of one egg is never worth the risk.