Why Does Ice Float?

Why Is Ice Less Dense Than Liquid Water?

Ice floats because the same amount of water takes up more room as a solid than as a liquid. More room for the same mass means lower density, and an object less dense than the fluid around it floats.^[7]

The extra room comes from how water molecules hold on to each other. Water is a polar molecule, so neighboring molecules weakly attract one another through hydrogen bonds.^[5] In ice, each interior molecule makes hydrogen bonds with four neighbors, accepting two and donating two.^[3] That fixed geometry builds a hexagonal lattice with a relatively open structure, including hexagonal channels running through it.^[3][4]

When ice melts, only some of those bonds break. The lattice collapses somewhat, the channels partly fill, and the molecules can get a little closer together on average.^[3][4] Most compounds go the other way and are denser as solids.^[6]

How Dense Are Ice and Water?

Density here is in grams per cubic centimeter (g/cm³), which is the same as grams per milliliter.

The gap between ice and water is far bigger than the gaps among the liquid rows. Divide the two 32°F values, 0.99987 ÷ 0.9167, and you get about 1.09. So freezing makes water take up about 9 percent more space, which matches the figure the USGS and a Penn State course give.^[1][6]

How Much of an Ice Cube Sits Below the Surface?

A floating object sinks until it has pushed aside its own weight of fluid.^[7] That leads to a tidy rule: the fraction under the surface equals the object's density divided by the fluid's density.^[7]

Using the table above for ice in water at the freezing point:

• Fraction below the surface: 0.9167 ÷ 0.99987 ≈ 0.917, or about 91.7 percent.
• Fraction above the surface: 1 − 0.917 = 0.083, or about 8.3 percent.
• For an ice cube 1 inch (25.4 mm) tall, floating flat: about 0.92 inches (23 mm) below and 0.08 inches (2 mm) above.

Warmer water barely changes the answer. In 50°F (10°C) water, 0.9167 ÷ 0.99975 still rounds to 0.917.

The USGS rounds more loosely and says about 10 percent of an ice cube or iceberg sits above the water line. It also notes that ice density varies because ice can contain air.^[1] Seawater adds another wrinkle. It has a density of about 1.027 at 32°F (0°C) and 3.5 percent salinity, and freshwater ice is about 0.92 by the same reference.^[8] That gives 0.92 ÷ 1.027 ≈ 0.90, so ice made from fresh water shows closer to a tenth of its volume in the ocean.

Why Is Water Densest Near 39°F (4°C)?

Two effects pull in opposite directions as liquid water warms from its freezing point. Warming breaks more hydrogen bonds, which lets the remaining open structure keep collapsing, so the volume shrinks. Warming also makes molecules move more, which pushes them apart.^[4]

Below the peak, the first effect wins. Above it, the second one does.^[4][5] The crossover comes at 3.98°C, or about 39.2°F.^[5]

The peak is gentle. The Open University gives about 999.86 kg/m³ at 32°F (0°C), almost 1000.0 at about 39°F (4°C) and 999.73 at 50°F (10°C).^[5] From 32°F to 39.2°F, density changes by only about 0.013 percent, using the USGS values above. That tiny difference is still enough to arrange a whole lake.

What Does This Mean for Frozen Lakes?

A lake cools in stages, as described in an Open University course:^[5]

1. In fall, surface water cools, gets denser and sinks, and warmer water rises to replace it.
2. This continues until the whole lake reaches about 39°F (4°C).
3. Further cooling makes the surface water less dense, so it stays on top instead of sinking.
4. The surface reaches 32°F (0°C) and ice forms there.

The ice then acts as an insulator between the water and the winter air, which keeps the water below from freezing solid and gives aquatic life a refuge.^[10][9] Penn State's course says deep water in large temperate lakes such as the Great Lakes stays near 39°F (4°C).^[6]

The USGS points out the alternative: if water were densest at its freezing point, cold surface water would sink in winter and a lake could freeze from the bottom up.^[1] Sinking ice would also leave the surface exposed to cold air, so the water would freeze much faster.^[4]

A frozen lid has a cost, though. Snow and ice block light and keep oxygen from entering from the air, so oxygen can fall to very low levels in a pond, and low oxygen is a common cause of winter fish kills.^[9]

Safety note: Floating ice is not the same as safe ice. The National Weather Service says ice is never 100% safe.^[14] It says to stay off ice under 2 inches thick and gives 4 inches as the minimum for walking, for new, clear ice only. White or "snow" ice is about half as strong, so it doubles those numbers. Stay off ice with cracks, holes or flowing water around its edges, or ice that has thawed and refrozen.^[13] Thickness also varies across a single body of water.^[15] As the Weather Service puts it, if you don't know, don't go.^[14]

Myths vs Facts

What We Still Can't Tell You

We ran no experiments for this article. The numbers come from published references, and they describe clean water and ice, not the cube in your glass. Bubbles, dissolved material and the liquid you drop it in can all shift the result, so treat 91.7 percent as a good estimate, not a measurement of your drink.

The simple "ice is an open lattice, liquid is a looser version" story is also a simplification. Researchers still debate the details of liquid water's structure. The classical picture has mostly four-coordinated molecules, while some X-ray spectroscopy work has been read as suggesting only two strong hydrogen bonds per molecule.^[11]

Lakes do not all follow the tidy four-step story either. A 2026 paper cites ample evidence of ice-covered lakes with bottom sediments cooler than 3.98°C (about 39.2°F), and notes that warmer bottom temperatures have been reported in places including Tibetan lakes and the Great Lakes.^[12] And nothing here can tell you whether a particular lake is safe to walk on today.