The Float Test: Why Fresh Eggs Sink and Old Eggs Float

Drop a fresh egg into a glass of water and it sinks. Drop an old egg in and it floats. This simple test has been used to assess egg freshness for centuries, and it works reliably because it measures a real physical change that happens to every egg over time: the growth of the air cell. As moisture and gases escape through the shell's pores, the internal volume available to liquid decreases and the air space expands. Since air is less dense than water, an egg with a large enough air cell becomes buoyant. ## The Air Cell: Formation and Growth When a freshly laid egg cools from the hen's body temperature (about 41 degrees Celsius) to room temperature, the contents contract slightly. The two shell membranes, which were pressed together at the blunt end, pull apart as the contents shrink, creating a small air cell. In a USDA Grade AA egg, this air cell is less than 3.2 millimeters deep. Grade A allows up to 4.8 millimeters. Anything larger is a lower grade. Once formed, the air cell grows throughout the egg's life as water vapor and carbon dioxide escape through the shell pores. The rate of growth depends on temperature, humidity, and whether the shell's cuticle is intact. Refrigerated eggs lose moisture more slowly than room-temperature eggs, which is why refrigeration extends shelf life. Unwashed eggs with the cuticle intact lose moisture more slowly than washed eggs, where the pores are unblocked. ## Interpreting the Float Test A fresh egg (1 to 7 days old) sinks flat to the bottom of a glass of water. The air cell is small and the overall density of the egg is slightly greater than water (approximately 1.08 g/cm3 for a fresh egg versus 1.00 g/cm3 for water). As the egg ages, moisture loss lowers the average density. At around 1 to 3 weeks, the blunt end of the egg begins to tilt upward while the pointed end remains on the bottom. The egg is increasingly buoyed at the end with the growing air cell. Beyond 3 to 4 weeks under refrigeration (or faster at room temperature), a sufficiently large air cell makes the whole egg buoyant enough to float. A floating egg is old. This does not necessarily mean it is unsafe to eat. "Old" and "spoiled" are different states. An old egg that floats may still be perfectly edible if it has been refrigerated continuously and the shell is intact. The smell test on cracking remains the definitive test for spoilage: hydrogen sulfide (rotten egg smell) indicates bacterial activity. If it smells fine, it is likely safe. ## Why Carbon Dioxide Loss Also Matters It is worth noting that moisture is not the only thing leaving the egg. Carbon dioxide dissolved in the white also escapes through the pores, particularly in the first few days after laying. This CO2 loss raises the pH of the white from about 7.6 at laying to 9.0 or higher within a week. Higher pH affects protein structure: the thick white thins, the chalazae weaken, and the yolk becomes less centered. The pH change also affects cooking: very fresh eggs have a lower pH, which affects foam stability and the texture of scrambled eggs and omelets. Professional bakers sometimes prefer eggs that are several days old for certain applications because the lower-viscosity whites blend more easily. For practical storage: store eggs in the refrigerator with the pointed end down. This keeps the air cell at the top and the yolk centered, away from the air cell and the bacteria that might penetrate from it. Most egg cartons are designed to store eggs pointed end down for this reason.