The Float Test Myth: Why Water Doesn’t Tell You If an Egg Is Good, Fertile, or Hatchable
(And Why “Water Candling” Is Even Worse)
Quick Answer: Does the Float Test Work for Hatching Eggs?
No. The float test only measures egg density, which changes as moisture is lost and the air cell grows. It does not determine fertility, embryo viability, or hatchability. Scientific research shows hatch success depends on factors like storage conditions, breeder nutrition, and incubation management—not whether an egg floats.
If you’ve spent any time around poultry groups, you’ve probably heard this advice:
“Put the egg in water—if it floats, it’s bad.”
“You can check fertility with a float test.”
“Water candling works better than a flashlight.”
It sounds simple. It feels convincing. The float test doesn’t test life—it tests air
But it’s not how eggs—or embryos—actually work.
Let’s walk through what the science really says.
What the Float Test Actually Measures
The float test is based on one thing: density.
A fresh egg is denser than water, so it sinks. As an egg ages, it slowly loses moisture and carbon dioxide through the shell. At the same time, the air cell inside the egg gets larger. That combination lowers the egg’s density, which makes it more likely to float.
This is well established in poultry science:
Eggs lose weight over time due to moisture loss through the shell
The air cell increases in size as that moisture is lost
Lower density leads to increased buoyancy
Research consistently shows this relationship between storage time, weight loss, and internal egg quality (Jones & Musgrove, 2005; Samli et al., 2005; Akyurek & Okur, 2009).
Here’s the key takeaway:
The float test is measuring age—not quality, not safety, and definitely not fertility.
What the Float Test Does Not Tell You
This is where things get misunderstood.
The float test cannot tell you:
Whether an egg is fertile
Whether an embryo is developing
Whether the egg will hatch
Whether the egg is contaminated
And in real-world breeding:
Eggs can float and still hatch
Eggs can sink and still fail
That’s because hatchability depends on completely different factors:
Embryo viability
Storage time and conditions
Temperature and humidity control
Handling stress (especially shipping)
Breeder nutrition
Studies on hatchability focus on these variables—not flotation—because they are what actually influence outcomes (Tona et al., 2003; Fasenko, 2007; Decuypere & Michels, 1992).
So when someone uses a float test to judge a hatching egg, they’re measuring the wrong thing entirely.
Let’s Talk About “Water Candling”
Now we get into something that’s not just inaccurate—but potentially harmful.
“Water candling” is the practice of putting eggs in water to try to see inside them or check development.
It’s become popular on social media because it looks interesting.
But from a biological standpoint, it introduces several risks.
Why Submerging Eggs Is a Problem
1. Eggshells Are Porous
Eggshells contain thousands of microscopic pores—typically between 7,000 and 17,000 (Board & Tranter, 1995).
These pores allow gas exchange, which is essential for embryo development.
They also allow moisture and bacteria to move across the shell under the right conditions.
When you submerge an egg in water, you increase the chance of liquid moving inward through those pores.
2. You Can Damage the Bloom
The outermost layer of the egg—the cuticle, or bloom—is a natural protective barrier.
It helps prevent bacteria from entering the egg.
Research shows this layer is vulnerable to moisture and handling (Leleu et al., 2011; De Reu et al., 2006).
Once that barrier is compromised, the egg becomes more susceptible to contamination.
3. Temperature Differences Matter
Even small temperature differences between the egg and the water can cause stress to a developing embryo.
Embryos are sensitive to temperature fluctuations, especially early in development (French, 1997; Lourens et al., 2005).
This kind of shock is not something you want to introduce unnecessarily.
4. Water Can Carry Bacteria
Water itself can act as a vehicle for bacteria—especially if it’s reused or not perfectly clean.
Studies have shown that wetting eggs can increase the risk of bacterial penetration, including Salmonella, particularly when temperature gradients are involved (Messens et al., 2005; Hutchison et al., 2003).
What Science and Hatcheries Actually Use
When researchers or hatcheries evaluate eggs, they rely on methods that are:
Non-invasive
Repeatable
Biologically relevant
These include:
Candling with light
Monitoring egg weight loss over time
Measuring specific gravity in controlled solutions
Haugh units (for internal quality)
Breakout analysis
You’ll notice what’s not on that list:
A bowl of water.
Practical Takeaways for Breeders
If your goal is better hatch rates, here’s what actually makes a difference:
Use proper candling with a light source
This allows you to monitor air cell development and embryo progress without introducing risk.
Control your storage conditions
Keep eggs around 55–65°F and turn them regularly if stored.
Focus on breeder nutrition
This has a significant impact on embryo viability and hatch success.
Limit unnecessary handling
Every time an egg is handled or exposed to new conditions, you introduce variables that can reduce hatchability.
The Bottom Line
The float test doesn’t test life—it tests air.
Water candling doesn’t reveal what’s happening inside the egg—it increases the risk of damaging it.
If you’re serious about improving hatch rates, these methods don’t help—and in some cases, they can hurt.
Final Thought
In poultry, there’s a big difference between something that looks like it works and something that’s grounded in biology.
The more you align your practices with how eggs and embryos actually function, the better your results will be.
Skip the shortcuts. Protect the egg. Focus on what truly impacts hatchability.
Jennifer Bryant of Bryant’s Roost and co-host of the Poultry Nerds Podcast specializes in hatching egg shipping, incubation success, and breeder management. Her work focuses on applying research-backed methods to improve hatch rates for small-scale and commercial poultry keepers.
Frequently Asked Questions
Does a floating egg mean it is bad?
No. A floating egg usually indicates it is older and has a larger air cell, but it may still be usable or even hatchable depending on other conditions.
Can you tell if an egg is fertile by putting it in water?
No. Fertility can only be confirmed through incubation and candling. Water tests do not reveal embryo development.
What is the best way to check a hatching egg?
The safest and most accurate method is candling with a light source to observe air cell size and embryo development.
Does water candling hurt eggs?
Yes, it can. Submerging eggs may damage the protective bloom, introduce bacteria, and expose embryos to temperature shock.
References
Tona, K. et al. (2003). Effects of egg storage time on hatchability. Poultry Science.
Fasenko, G.M. (2007). Egg storage and embryo development. Poultry Science.
Decuypere, E. & Michels, H. (1992). Incubation conditions and embryonic growth.
Jones, D.R. & Musgrove, M.T. (2005). Eggshell conductance and moisture loss.
Samli, H.E. et al. (2005). Effects of storage time on egg quality.
Akyurek, H. & Okur, A.A. (2009). Storage impacts on internal egg quality.
Board, R.G. & Tranter, H.S. (1995). The microbiology of eggs.
De Reu, K. et al. (2006). Eggshell contamination and bacterial penetration.
Leleu, S. et al. (2011). Cuticle quality and egg protection.
Messens, W. et al. (2005). Salmonella penetration in eggs.
Hutchison, M.L. et al. (2003). Egg washing and microbial risks.
French, N.A. (1997). Temperature effects on embryo development.
Lourens, A. et al. (2005). Incubation temperature and embryo physiology.