Ice takes up more room than the liquid water it freezes from. That single fact is why an ice cube floats, why a chunk of an iceberg keeps most of its bulk hidden underwater, and why a frozen pond still has liquid water, and living fish, sitting under the ice in January.
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−Water is one of a short list of common substances where the solid form is less dense than its liquid, and that quirk does real work well past a drink getting colder.
It sets how deep a water line has to sit underground to survive winter. It also decides whether a garden pond needs help keeping anything alive until spring.
The short version
- Ice is about 9 percent less dense than liquid water, because hydrogen bonds lock it into an open lattice.
- About 87 percent of an iceberg sits below the waterline in seawater.
- Buried water lines must sit below the local frost depth, which ranges from near zero to more than 72 inches.
- Ponds rarely freeze solid, but full ice cover can cause winterkill from low oxygen.
- Equipment above the frost line, like an outdoor softener or garage filter, needs its own protection.
What makes ice less dense than the water it came from
The U.S. Geological Survey pegs water’s maximum density at 39.2 degrees Fahrenheit, 4.0 degrees Celsius, the point where a cubic centimeter weighs almost exactly one gram.
Cool it past that and something counterintuitive happens. Instead of continuing to shrink the way most liquids do, water starts expanding again, because hydrogen bonds begin locking its molecules into an open, six sided lattice that spaces them farther apart rather than packing them tighter.
By the time it fully freezes, that lattice leaves ice about 9 percent less dense than the liquid water sitting right next to it. That is the same figure the USGS cites for the density drop at the freezing point.
An ice cube in a glass of tap water floats with roughly 91 to 92 percent of its volume below the surface, a direct result of that 9 percent gap. Only a thin cap breaks the waterline.
It looks like a small amount, and in a glass it is. But the same ratio scales up to something much bigger once it is sitting in the ocean instead of a kitchen.
How much of an iceberg actually sits above the water
Icebergs make the effect visible at a scale nobody can miss. Here is how the densities compare:
| Substance | Density |
|---|---|
| Liquid water at 39.2°F (4.0°C) | Almost exactly 1 gram per cubic centimeter |
| Compressed glacial ice | Close to 0.90 grams per cubic centimeter |
| Seawater | Closer to 1.03 grams per cubic centimeter, because of its dissolved salt |
The U.S. Coast Guard’s Navigation Center puts the result at about seven-eighths of an iceberg’s volume, close to 87 percent, sitting below the waterline. What shows above the surface really is the smaller share.
It is denser seawater, not a different ice, that pushes the visible tip down slightly compared to an ice cube floating in a glass of plain tap water.
The one exception: heavy water
There is one well documented exception worth knowing, mostly as a curiosity rather than anything a homeowner will ever run into.
Heavy water, made with deuterium in place of ordinary hydrogen, runs about 10.6 percent denser than regular water. Ice frozen from it actually sinks in a normal glass instead of floating.
Nobody’s tap water behaves this way. It is the exception that confirms the rule, since floating tracks density, not simply the fact that something has frozen.
Why this matters underground, not just in a glass
The same density gap explains a decision plumbers and local building departments make long before the first freeze of the season.
How deep a water line has to be buried
The International Residential Code’s water service section, P2604, does not set one national burial depth for supply lines. It requires a water line to sit deep enough to avoid freezing based on the frost depth already established for that jurisdiction.
The number swings hard by region, from close to zero inches in southern Florida to more than 72 inches around International Falls, Minnesota.
A buried line survives winter for the same reason a lake does not freeze solid: the coldest water, and eventually the ice, stays near the surface rather than sinking to where the pipe sits.
Why a frozen pipe bursts
A supply line that does freeze still fails for a specific reason. The ice plug traps liquid water between itself and a closed fixture, and pressure builds in that trapped section.
The pipe wall eventually gives out at its weakest point rather than the ice itself splitting the material. It is the same pressure buildup that makes a pipe burst once water freezes solid inside it.
Filters and softeners above the frost line
Equipment sitting above that line does not get the same protection automatically. A whole house filter or softener mounted in an unheated garage, a crawl space, or right at an outdoor well head sits exactly where a shallow, under-code line would freeze first.
Most residential softeners carry their own manufacturer temperature limits for this reason. If you are shopping for a whole house water softener for well water in a cold climate, confirm the unit’s rated operating range before buying.
The mineral tank and control valve sitting outdoors get none of the protection a properly buried pipe gets for free. Insulation around the housing, heat tape on exposed fittings, or simply relocating the unit indoors matters as much as burial depth does for the plumbing feeding it.
Why a pond does not freeze solid on its own
The same density curve keeps a pond, a lake, or an unheated stock tank from freezing all the way to the bottom. It is not automatic in the sense that nothing bad can ever happen. Here is how it plays out:
- As surface water cools in the fall, it sinks once it passes that 39.2 degree densest point, pushing relatively warmer water up.
- This continues until the whole column reaches roughly the same temperature.
- Only after that does the surface keep cooling on its own, forming ice that floats and insulates everything underneath rather than sinking and freezing the water below layer by layer.
Fish and other cold blooded animals settle into the warmer water near the bottom and slow down for the season instead of dying, a state closer to torpor than true hibernation.
The winterkill risk
The insulation cuts both ways, though. Ice sealing the surface also blocks oxygen exchange with the air and cuts off the light submerged plants need for photosynthesis.
A pond that stays fully iced over for an extended stretch can suffer what pond managers call winterkill, an oxygen crash that suffocates fish even though nothing ever actually froze solid.
A small opening kept in the ice, through an aerator or a floating de-icer, matters more for oxygen exchange than for temperature. The water underneath was never in real danger of freezing in the first place.
Does salty or hard water float and freeze the same way
Dissolved salt changes the numbers slightly without changing the underlying physics. Seawater’s dissolved minerals lower its freezing point below the 32 degree mark pure water uses.
That is part of why nobody safely drinks seawater straight from the ocean even in an emergency, since the salt causes problems long before the cold does.
A well running high in dissolved solids behaves the same way on a much smaller scale, needing a slightly harder freeze than pure water before ice starts forming at all.
The difference is not large enough to change how deep a plumber should bury a line. It is one more reason a real water test, not a guess, should drive any decision about treating a well that tastes off.
What actually protects your pipes
The decisive point for anyone dealing with either a pipe or a pond is the same one. Ice floating protects whatever already sits below the waterline or below the frost line automatically, without anyone doing anything, but it does nothing for equipment sitting above that line.
A buried water service pipe survives winter because it was never installed where ice forms in the first place. A lake’s fish survive because the coldest water rises and freezes at the top instead of the bottom.
Anything installed above that line needs its own protection:
- An exposed hose bib
- An outdoor softener
- A whole house filter in an uninsulated garage
The same physics that keeps a fish alive under three inches of ice will crack a housing that was never buried or insulated deep enough to begin with.
Jay
Jay is a health and wellness enthusiast with expertise in water quality and nutrition. As a knowledgeable advocate for holistic well-being, Jay successfully manages Type 2 Diabetes through informed lifestyle choices. Committed to sharing reliable and authoritative insights, Jay combines firsthand experience with a passion for enhancing health."