The volume of water increases when it freezes, meaning a measured cup of ice will always yield a smaller volume of liquid water once it melts. Understanding the science behind this conversion provides a clear answer, though real-world variables can introduce slight differences.
The Standard Conversion Ratio
The precise, theoretical conversion is governed by the difference in density between liquid water and solid ice. One cup of solid ice, assuming no air pockets are present, melts down to approximately 0.92 cups of liquid water. This ratio is derived from the average density of ice being about 92% that of liquid water at their respective freezing and melting points.
One full cup of ice will produce roughly seven-eighths of a cup of water, or about 87.5% to 92% of the original volume. The conversion factor of 0.92 is the scientific standard for calculating the liquid volume from a solid ice volume, representing the ideal scenario of pure, solid H₂O.
The Physics of Volume Change
Ice takes up more space than liquid water due to its molecular structure. Water molecules are polar, meaning they have a slight positive charge on the hydrogen side and a slight negative charge on the oxygen side. These polar molecules form temporary connections called hydrogen bonds in the liquid state.
When water cools to its freezing point, these hydrogen bonds lock into a highly organized, hexagonal crystalline structure. This rigid, open lattice arrangement holds the water molecules farther apart than they are when they are freely moving in the liquid state. This increased spacing explains why ice has a lower density than water, allowing it to float.
The volume increases by about 9% when liquid water turns into ice at standard atmospheric pressure. This physical property dictates the standard conversion ratio. The difference in volume is a direct consequence of the molecular geometry that forms when the water crystalizes.
Real-World Measurement Variables
While the scientific conversion is fixed, the volume of water obtained from a cup of ice in a real-world scenario is less predictable due to practical factors. The most significant variable is the amount of air space within the measured volume of ice. A cup of ice cubes, for instance, contains substantial gaps between the individual cubes, meaning the cup is not filled entirely with solid water.
Loosely packed ice cubes result in a melted water volume much less than the 92% theoretical maximum. Crushed ice or fine ice shards pack together more tightly, reducing air space and yielding a melted volume closer to the ideal 92% ratio. The shape and size of the ice pieces are major determinants of the final liquid volume.
This variability has practical implications, particularly in cooking or tracking hydration. For a recipe requiring a specific volume of melted water, tightly packed crushed ice is a more accurate measure than loosely packed cubes. When tracking water intake, the total hydration volume is the actual liquid volume, which will be slightly less than a full cup.