Density of Ice: 917 kg/m³ — Why Ice Is Less Dense Than Water
Ice has a density of about 917 kg/m³, roughly 8.3% lower than liquid water at 999.84 kg/m³. It is one of nature's most important density surprises: the same substance becomes lighter when it freezes.
Almost every other material is denser as a solid than as a liquid, and water is the famous exception. This page explains the value, the molecular mechanism behind it, and why the effect matters for life on Earth. For live water comparisons, use the liquid density calculator.
Key values
Ice Density: Key Values
kg/m³
917 kg/m³
Ordinary ice (Ice Ih) at 0°C
g/cm³
0.917 g/cm³
Standard reference value
lb/ft³
57.2 lb/ft³
U.S. reference
These values refer to ordinary hexagonal ice (Ice Ih), the form that occurs naturally on Earth. At least 18 other crystalline forms of ice exist under extreme pressure or temperature conditions.
Ice vs water
Ice Density vs Water Density
Ice and water are two phases of the same substance (H₂O), but their densities are significantly different. That difference directly determines whether ice floats on water.
For the liquid phase, compare the full density of water reference.
| Ice (0°C) | Liquid Water (0°C) | |
|---|---|---|
| Density | 917 kg/m³ | 999.84 kg/m³ |
| g/cm³ | 0.917 | 0.9998 |
| lb/ft³ | 57.2 | 62.4 |
| Volume of 1 kg | 1,090 cm³ | 1,000 cm³ |
| Behaviour in water | Floats | — |
A 1 kg block of ice occupies about 1,090 cm³, while 1 kg of liquid water occupies only 1,000 cm³.
Water expands by about 9% as it freezes, which is why a sealed container filled with water can rupture after freezing.
Molecular structure
Why Is Ice Less Dense Than Water?
For almost every substance on Earth, the solid phase is denser than the liquid phase. When a material cools and solidifies, its molecules slow down, pack more tightly, and occupy less space. Iron, gold, alcohol, and most other materials all follow this pattern. Water is a striking exception. The baseline idea is explained in what is density.
Water molecules (H₂O) are polar: the oxygen end carries a slight negative charge and each hydrogen end carries a slight positive charge. This polarity allows water molecules to form hydrogen bonds with their neighbours — each molecule can bond with up to four others. In liquid water, these bonds are constantly forming and breaking, keeping molecules relatively close together.
When water freezes, the hydrogen bonds lock into a fixed hexagonal lattice structure. This lattice is more open than the disordered arrangement of liquid water — the molecules are held at specific angles and distances that create a rigid but spacious framework. The result is that ice occupies about 9% more volume than the same mass of liquid water, giving it a lower density.
This structural quirk has profound consequences. Because ice floats, frozen lakes and rivers develop an insulating ice layer on top rather than freezing solid from the bottom up. Aquatic life survives winters beneath that ice layer. If water behaved like most substances, lakes would freeze from the bottom, eliminating most freshwater ecosystems in cold climates. Related concepts include buoyancy and density.
Temperature effect
Ice Density at Different Temperatures
Ice density increases slightly as temperature drops because thermal expansion weakens. The change is much smaller than the temperature effect in liquid water.
Across ordinary freezing conditions, the numbers move by only a few kg/m³ rather than the sharp density difference between ice and liquid water.
| Temperature | Ice Density | Notes |
|---|---|---|
| 0°C | 917 kg/m³ | Melting point, standard reference |
| −10°C | 918 kg/m³ | Slight increase as temperature drops |
| −20°C | 919 kg/m³ | Typical freezer temperature |
| −40°C | 922 kg/m³ | Extreme cold climate reference |
| −78.5°C | 930 kg/m³ | Dry ice temperature (CO₂, for comparison) |
Compare: liquid water at 0°C is 999.84 kg/m³ and reaches its maximum density of 999.97 kg/m³ at 3.98°C. See the full water density temperature table → density of water.
For broader material comparisons, open the density table.
Consequences
Real-World Consequences of Ice Floating
Icebergs and polar ice
About 90% of an iceberg sits below the water surface because ice is only slightly less dense than sea water (917 vs ~1,025 kg/m³).
The visible tip represents roughly 10% of the total mass — the origin of the phrase "tip of the iceberg."
Burst pipes in winter
When water freezes inside a pipe, it expands by about 9%. A sealed pipe has no room to accommodate this expansion, so pressure builds until the pipe fractures.
This is one of the most common causes of water damage in cold climates.
Lake stratification and aquatic life
Ice forms on the surface of lakes rather than the bottom, acting as an insulating lid.
Water beneath the ice stays liquid and remains near 4°C — dense enough to support aquatic life through winter. Bottom-up freezing would make most cold-climate lakes uninhabitable.
The water cycle and climate
Floating ice reflects sunlight back into space (high albedo), helping regulate Earth's temperature.
Melting sea ice changes ocean salinity and density gradients, which in turn affect ocean circulation patterns and climate systems globally.
FAQ
Frequently Asked Questions
What is the density of ice in kg/m³?
Ordinary ice (Ice Ih) at 0°C has a density of 917 kg/m³. This is about 8.3% less than liquid water at the same temperature (999.84 kg/m³), which is why ice floats. As temperature drops below 0°C, ice density increases slightly — reaching about 922 kg/m³ at −40°C.
Why is ice less dense than water?
When water freezes, hydrogen bonds lock the molecules into a hexagonal lattice structure that is more open and spacious than the disordered arrangement in liquid water. This means the same mass of water occupies about 9% more volume as ice, giving ice a lower density. Almost all other substances are denser as solids than as liquids — water is a rare and important exception.
Why does ice float on water?
Ice floats because its density (917 kg/m³) is lower than liquid water (999.84 kg/m³ at 0°C). Any object less dense than the fluid it is placed in will float — this is Archimedes' principle. Because ice is about 8.3% less dense than water, roughly 92% of a floating ice cube sits below the surface.
Does ice density change with temperature?
Yes, but only slightly. As ice gets colder, it contracts very gradually and becomes marginally denser — from 917 kg/m³ at 0°C to about 922 kg/m³ at −40°C. This change is much smaller than the dramatic density shift that occurs when liquid water freezes at 0°C.
What happens when water expands as it freezes?
Water expands by approximately 9% in volume when it freezes. In open containers this is harmless, but in sealed pipes or rock crevices the expansion generates enormous pressure — enough to burst metal pipes or split rock faces. This freeze-thaw cycle is one of the primary mechanisms of physical weathering in cold climates.