Is This Egg Fresh? A Density Test You Can Trust

Almost everyone has been told that a floating egg is rotten. It is not, and the United States Department of Agriculture says so plainly: an egg floats when its air cell has grown large enough to make it buoyant, which means the egg is of poor quality but may be perfectly safe to use. That gap between what people believe and what the test actually measures is why the egg float test science is worth doing properly, with four eggs of known ages, a ruler and a scale. The test is genuinely trustworthy. Just not for the thing people trust it for.

Key Takeaways

• You will float four eggs of known different ages in plain water, then crack each one and measure the white and the yolk.
• The concept is density and Archimedes’ principle: as the air cell grows, the egg’s average density falls below that of water.
• Good for ages 5 to 11. The youngest can sort eggs by how they sit; older kids can calculate density.
• The misconception to break: floating does not mean rotten. It means old. The real spoilage test is your nose.
• All the eggs are food. Cook them, except any that smells off when cracked.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Eggs of different known ages (contains egg)	4: bought today, one week, three weeks, five weeks	Supermarket, over several weeks	Buy four at once and start the experiment five weeks later, dating each as you go
Tall glass or jar	1, at least 15 cm deep	Kitchen cupboard	A measuring jug; the egg needs to float clear of the base
Plain tap water	about 600 mL	Tap	Plain water only. Do not add salt, which makes everything float
Pencil	1	Desk	For dating the shells; a marker works but bleeds through
Kitchen scale	1, to 1 g	Kitchen	Essential for the density calculation
Ruler	1	Pencil case	For measuring the white and the yolk height
Four saucers	4	Kitchen	Flat white plates are better than bowls
Notebook	1	Desk	Four eggs, four sets of numbers

Time: setup 5 weeks ahead if you are dating fresh eggs, 20 min to run, 15 min to measure Cost: four eggs Mess level: low, then medium once you start cracking them.

Do the Experiment

Step 1 — Date the eggs in pencil as you buy them

Write the purchase date on each shell in pencil, in the fridge, as you go. This is the only genuinely hard part of the experiment, because it needs planning five weeks ahead. If you cannot wait, buy four eggs now and run it as a five-week project with a float test each week.

Step 2 — Weigh each egg and record it

A large egg is 55 to 65 g. Write down each egg’s mass before it goes anywhere near water, because you need it for both the density and the albumen quality calculation.

Step 3 — Float them one at a time

Fill the glass with plain water to 12 cm or so, and lower each egg in gently with a spoon. Record how it sits: flat on the bottom, tilted, standing on its small end, or floating. Take a photograph of each.

Step 4 — Measure the volume by displacement

Fill a measuring jug to a known level, lower the egg in, read the new level, and subtract. A large egg is 50 to 60 cm³. Mass divided by volume is density, and this is the number the whole float test is secretly reporting.

Step 5 — Crack each onto a saucer and measure

One at a time, onto its own saucer. Measure the diameter the white spreads to, and the height of the thick white next to the yolk with a ruler held vertically. Also note the yolk’s height and whether it holds its dome. Then smell each one before you cook them.

What you should see: The egg bought today sinks immediately and lies flat on its side on the bottom. The one-week egg sinks but settles at a slight tilt. The three-week egg stands upright on its narrow end, still touching the bottom. The five-week egg floats, usually with a small part of the shell breaking the surface. Cracked open, the progression is just as clear: today’s egg has a tall domed yolk and a thick white that holds a compact oval about 7 cm across, while the five-week egg has a flat yolk that breaks easily and a thin white spreading to 12 cm or more. Density falls from about 1.08 g/cm³ for the fresh egg to below 1.00 for the floater. And here is the part that matters: unless an egg smells bad when cracked, all four are still food.

What’s Actually Going On in the Egg Float Test Science

An egg is laid at about 41 °C and, according to USDA guidance, has no air cell at all at that moment. As it cools, the shell and its contents contract at different rates, and that difference pulls a small pocket of air in between the two shell membranes, usually at the wide end. That pocket is the air cell, and from the moment it forms it only gets bigger.

The reason it grows is that an eggshell is not sealed. Hincke and colleagues, describing eggshell structure and composition in 2012, set out a mineralised shell penetrated by thousands of pores. Water vapour and carbon dioxide leave through those pores, and air comes in to replace them. USDA puts the consequence plainly: moisture and carbon dioxide in the white evaporate through the pores, more air penetrates the shell, and the air cell becomes larger.

That single process explains everything you measured. Mass falls, because water is leaving. Volume stays almost constant, because the shell is rigid. So density falls. When the average density of the whole egg drops below the density of water, it floats. The float test is a density measurement using Archimedes’ principle, and it is measuring how much water has left the egg since it was laid.

The cracked-open measurements come from the same process by a different route. Carbon dioxide leaving the white raises its pH substantially, from close to neutral at lay toward strongly alkaline over a few weeks. That change breaks down the protein structure that makes thick albumen thick, so the white thins and spreads. The yolk, meanwhile, absorbs water from the white across the vitelline membrane, which makes it larger, flatter and much easier to break. USDA describes exactly this: the white thins and loses its thickening and leavening power, the yolk flattens, and the chalazae weaken so the yolk moves off centre.

The poultry industry turned the albumen measurement into a number. Eisen, Bohren and McKean analysed the Haugh unit as a measure of egg albumen quality in Poultry Science in 1962, and it is still the standard: a calculation combining the height of the thick white with the weight of the egg. A ruler and a kitchen scale get you a reasonable approximation of the same thing, which is a nice example of an industrial quality metric being accessible at home.

Now the honest part, and the whole reason this article exists. USDA’s position is that an egg floating in water means poor quality but may be perfectly safe to use, and that you should crack it into a bowl and check for off-odour or unusual appearance before deciding. Spoilage is bacterial, and bacteria are not what is making the air cell grow. An egg can be old and fine, and a young egg can be contaminated. So the float test is a good age test and a bad safety test, and treating it as a safety test throws away edible food while providing false reassurance in the opposite direction.

How to Explain It to Your Kid

Ages 5-8: The egg slowly gets a bubble inside

Tell them an egg has tiny holes in its shell, too small to see, and that water sneaks out of them slowly while the egg sits in the fridge. Air comes in instead, so the egg gets lighter and lighter until it can float. Let them line the four eggs up by how they sit in the glass. Then say the important bit out loud: floating means old, not bad.

Ages 9-12: Calculate density

Give them mass divided by volume. Measure both, do the division, and watch the number cross 1.00 g/cm³ exactly where the behaviour changes from sinking to floating. That is a prediction they can make before the last egg goes in the water, which is far more satisfying than watching it. Water’s density being 1.00 g/cm³ turns out to be useful rather than arbitrary.

Ages 13+: pH, protein and a real industry metric

Explain the carbon dioxide story: it leaves the white, so carbonic acid concentration falls, so pH rises toward 9, so the protein complex giving thick albumen its structure comes apart. Then introduce the Haugh unit, have them measure albumen height with a ruler, and discuss why industry pairs it with egg weight. Then set the real investigation: two eggs of the same age, one refrigerated and one at room temperature.

The question to ask: “If I put a brand-new egg in salty water instead of plain water, would it float? And what does that tell you about what the test is really measuring?”

Why Teaching This Early Pays Off

Density is the first quantitative idea in physical science that children can genuinely measure at home, and it is usually taught with a list of materials to memorise. Here it does real work: it answers a question the household actually has. A kid who has worked out that mass over volume crossing 1.00 is what makes the egg float has met the concept doing its job, and the same reasoning explains why seven liquids stack in a density column.

The more valuable lesson is about a belief being almost right. “Floating eggs are bad” is not nonsense: it correlates with age, and very old eggs are more likely to be unpleasant. Smith, diSessa and Roschelle argued in 1994 that misconceptions are typically partial knowledge in transition rather than errors to be deleted, and this one is a textbook case. The fix is not “you’re wrong”, it is “that measures age, and here is how to measure safety instead”. A child who learns to ask what a test actually measures has acquired something far more portable than a fact about eggs.

Keep the claims proportionate. Running this once will not make anyone numerate, and the experiment is slow because it depends on eggs aging. What it does deliver is a genuinely useful household skill, a correct understanding of a widespread piece of wrong advice, and a measurement they made themselves that crossed a threshold they predicted.

Before You Start: What Parents Should Know

Best age	5 to 11. Younger children sort and observe. From about nine, the density calculation is the point.
Supervision	Minimal. An adult should handle the cracking and the smell check, and decide what gets cooked.
Real hazards	None beyond normal kitchen care. Raw egg can carry Salmonella, so wash hands and surfaces after handling, and do not let anyone taste raw egg. USDA also notes that eggs should not be washed at home, because wash water can be drawn in through the pores.
Don’t	Do not add salt to the water to “make it work better”. Salt water is denser, so it floats fresh eggs too, and you have destroyed the measurement. Plain tap water only.

On eating them: yes, all four, with one condition. Crack each egg onto its own saucer and smell it. A spoiled egg has an unmistakable sulphurous smell and USDA describes that odour as the reliable indicator, whether raw or cooked. If it smells normal, cook it thoroughly and eat it. The five-week egg will make poor fried eggs, because the white spreads, but it scrambles and bakes perfectly well, which is itself a useful thing for a child to learn about matching ingredients to jobs.

When It Doesn’t Work

• All four eggs sank. They were all fresher than you thought. Supermarket eggs can be several weeks old at purchase or only a few days, and the printed date tells you the pack date rather than the lay date. Extend the experiment and retest weekly.
• All four floated. You added salt, or you are using a very salty mineral water. Plain tap water, nothing else.
• The old egg floated but looked and smelled fine. That is the expected result, not a failure, and it is the single most important observation in the whole experiment. Age and spoilage are different things.
• The albumen measurements were all similar. You measured the thin outer white rather than the thick white next to the yolk. Hold the ruler vertically about 1 cm from the yolk, in the raised part.
• An egg cracked in the glass. Lower eggs with a spoon rather than dropping them, and use a tall jar rather than a narrow one. If one cracks in the water, discard it and wash the glass with hot soapy water.

Take It Further

Test refrigeration properly. Buy six identical eggs. Put three in the fridge and leave three in a cupboard at room temperature. Float and weigh one from each group at one, two and three weeks. The room-temperature eggs lose mass faster and reach the floating stage much sooner, which quantifies exactly what refrigeration is buying you. If you start with refrigerated eggs, keep them refrigerated, because USDA notes that a cold egg left out can sweat, which helps bacteria move into the egg.

Track one egg over five weeks. Pick a single egg, and each week weigh it, measure its volume by displacement, calculate its density and note how it sits in the glass. Plot density against week. It is a straight-ish line crossing 1.00, and watching a prediction come true in week four is a different experience from comparing four eggs at once. It pairs neatly with spinning an egg to tell raw from boiled, which measures something else entirely about the same object.

What to Watch For Over the Next 3 Months

• Week 4: They float an egg before cooking it, unprompted, and correctly describe what the result means.
• Month 2: The word “density” appears in a context you did not set up, usually about something floating or sinking in the bath.
• Month 3: They challenge a piece of household advice by asking what it actually measures. That is the transferable skill, and it started with an egg standing on its end.

Frequently Asked Questions

Does a floating egg mean it has gone bad?

No. USDA states that an egg floats when its air cell has enlarged enough to make it buoyant, which means poor quality but possibly perfectly safe. Crack it into a bowl and check for an off-odour or unusual appearance before deciding.

What actually makes an egg float over time?

Water vapour and carbon dioxide escape through thousands of tiny pores in the shell, and air replaces them. The egg loses mass while its volume stays fixed, so its density falls. Once it drops below the density of water, it floats.

How do I tell whether an egg is safe to eat?

Crack it onto a plate and smell it. A spoiled egg smells unmistakably sulphurous, raw or cooked. Also look for unusual colour or appearance. That check works regardless of what the egg did in the water.

Why do old eggs spread out in the pan?

Because carbon dioxide leaving the white raises its pH toward strongly alkaline, which breaks down the protein structure holding the thick albumen together. The white thins and spreads, and the yolk flattens as it absorbs water from the white.

Should I refrigerate eggs?

If they were sold refrigerated, yes, and keep them that way. USDA notes that once eggs have been refrigerated they must stay refrigerated, because a cold egg left at room temperature can sweat, which helps bacteria move through the shell.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. U.S. Department of Agriculture, Food Safety and Inspection Service. “Shell Eggs from Farm to Table.” https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/eggs/shell-eggs-farm-table
2. Eisen, E. J., Bohren, B. B., & McKean, H. E. (1962). “The Haugh unit as a measure of egg albumen quality.” Poultry Science, 41(5), 1461-1468. https://doi.org/10.3382/ps.0411461
3. U.S. Department of Agriculture, Agricultural Marketing Service. “Egg Grading Manual.” https://www.ams.usda.gov/grades-standards/egg-grading-manual
4. Hincke, M. T., Nys, Y., Gautron, J., Mann, K., Rodriguez-Navarro, A. B., & McKee, M. D. (2012). “The eggshell: structure, composition and mineralization.” Frontiers in Bioscience, 17, 1266-1280. https://doi.org/10.2741/3985
5. Smith, J. P., diSessa, A. A., & Roschelle, J. (1994). “Misconceptions reconceived: a constructivist analysis of knowledge in transition.” Journal of the Learning Sciences, 3(2), 115-163. https://doi.org/10.1207/s15327809jls0302_1
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165
7. White, R., & Gunstone, R. (1992). Probing Understanding. Falmer Press.