A bad egg announces itself clearly if you know what to look for. The simplest check takes seconds: crack it onto a plate and smell it. A rotten egg produces an unmistakable sulfur smell caused by hydrogen sulfide gas, and no amount of second-guessing will make you miss it. But there are also ways to check before cracking, and signs that are more subtle than outright rot.
The Float Test
Fill a bowl or glass with plain cold water and gently lower the egg in. A fresh egg sinks to the bottom and lies flat on its side. An egg that’s a few weeks old will sink but stand upright on one end. An egg that floats to the surface is old enough that you should crack it open and inspect it before using it.
Here’s why this works: eggshells are porous, and over time, moisture escapes while air seeps in through thousands of tiny pores. This gradually enlarges the air cell inside the wider end of the egg, making it lighter relative to its size. Eventually, the egg displaces more water than it weighs, and it floats. A floating egg isn’t automatically dangerous, but it’s old enough to warrant a closer look and a sniff test before cooking.
What a Fresh Egg Looks Like
The most reliable check is also the most direct. Crack the egg onto a flat plate or into a bowl and look at it before it goes into anything else.
A fresh egg has a bright yellow or orange yolk that sits up tall and round, surrounded by a thick white that holds its shape and doesn’t spread far across the plate. As eggs age, the proteins in the white break down, so the white becomes thinner and runnier. The yolk membrane weakens too, making the yolk flatten out instead of sitting up in a dome. An older egg with a flat yolk and watery white is still safe to eat, it just won’t poach or fry as nicely.
A bad egg looks distinctly different. The yolk may be discolored, the white appears murky or cloudy with no clear separation between yolk and white, and the overall appearance is undefined and off. If anything looks or smells wrong, toss it.
The Smell Test
Bacterial decomposition inside a spoiled egg produces hydrogen sulfide, the same compound responsible for the classic “rotten egg” smell. Humans can detect this gas at extraordinarily low concentrations, as little as 0.0005 parts per million in air. In practical terms, this means your nose is one of the most sensitive instruments you have. If an egg smells sulfurous or just “off” when you crack it, that’s all the information you need. Throw it away.
A fresh egg has almost no smell at all, or a very faint neutral odor. Any noticeable unpleasant smell, even a mild one, means the egg has started to spoil.
Dates on the Carton
Egg carton dates are less useful than most people assume. The USDA is clear on this point: except for infant formula, dates on food packaging indicate quality, not safety. A “sell-by” or “best by” date is not a federal requirement for eggs, though some states mandate it. An egg past its printed date can still be perfectly safe if it’s been stored properly and shows no signs of spoilage.
The more useful number on many egg cartons is the three-digit pack date, sometimes called the Julian date. This tells you exactly when the eggs were packaged. January 1 is 001, February 1 is 032, and December 31 is 365. If you see “137” stamped near the expiration date, those eggs were packed on May 17. This gives you a concrete reference point for how old your eggs actually are, regardless of what the sell-by date says.
How Long Eggs Last
Refrigerated eggs have a surprisingly long shelf life. According to USDA research, refrigeration extends egg shelf life from about 21 days to roughly 15 weeks. That’s nearly four months at 40°F or below, which is standard refrigerator temperature.
The reason American eggs need refrigeration in the first place is that commercial processing washes them, removing a natural protective coating called the cuticle (or “bloom”) that seals the shell’s pores. Without this barrier, bacteria can enter more easily and moisture escapes faster. Unwashed eggs, common in many other countries, can keep for weeks at room temperature because the cuticle stays intact. But once an egg has been refrigerated, keep it refrigerated. Moving it to room temperature causes condensation on the shell, which can actually help bacteria get through the pores.
When an Egg Looks Fine but Isn’t
The tricky thing about salmonella is that a contaminated egg can look, smell, and taste completely normal. Salmonella bacteria can be present inside the egg before the shell even forms, deposited by an infected hen. You can’t detect this through any home test.
Salmonella infection from eggs typically causes diarrhea, fever, and stomach cramps starting 6 hours to 6 days after exposure. Most people recover in 4 to 7 days without treatment. Young children, adults over 65, and people with weakened immune systems face higher risk of severe illness. A 2024 outbreak linked to eggs sickened 93 people across 12 states, with about 39% of those cases requiring hospitalization.
The practical protection here is cooking temperature. Egg dishes reach safety at an internal temperature of 160°F, which kills salmonella. This applies to scrambled eggs, quiches, casseroles, and any recipe where eggs are mixed into a cooked dish. For recipes that traditionally use raw or lightly cooked eggs (homemade mayonnaise, Caesar dressing, eggnog), you can heat the eggs in liquid from the recipe over low heat, stirring constantly, until the mixture hits 160°F.
Quick Reference for Checking Eggs
- Float test: Sinks and lies flat means fresh. Sinks but stands upright means older but likely fine. Floats means inspect carefully before using.
- Visual check: Tall round yolk and thick white means fresh. Flat yolk and runny white means old. Murky, discolored, or undefined appearance means discard.
- Smell: No smell or neutral odor means fine. Any sulfur or unpleasant odor means throw it out immediately.
- Carton date: Look for the three-digit pack date rather than relying solely on the sell-by date. Eggs refrigerated consistently are typically good for up to 15 weeks from packing.