A fluid, whether liquid or gas, pushes objects toward floating or rising, and that tendency is called buoyancy. Average density decides the outcome: an object floats when its density, averaged over its whole body, is lower than that of the fluid. Density is the amount of mass in a given volume, and fresh water, as of November 2026, comes to roughly 1 gram in every cubic centimeter. Clay offers a simple test. A solid lump sinks, but the same clay, once shaped like a boat, floats, because a heavy substance floats once it is shaped hollow and filled with air. A big ship floats for the same reason: with all the air inside counted, its average density comes out lower than water's. A ship rides higher when empty than when it carries cargo.
Archimedes' principle covers any body in a fluid that is at rest. The fluid pushes it upward with a force equal to what the displaced fluid weighs. To see why, picture the object gone, with water filling the space it occupied; that water was supported by the water around it, so the buoyant force acting on the object has to match the weight of that water. An object afloat sinks only far enough to push aside its own weight in water, and a launched ship goes down into the sea by just that amount. The cause is pressure. Pressure rises with depth, so the bottom of an object is pushed up harder by the water than its top is pushed down. On Floating Bodies, by Archimedes, is the oldest known work on hydrostatics, and he is recognized as founding that field. He lived around 287 to 212 BCE in Syracuse, Sicily.
Around 2,000 years back, Vitruvius, a Roman architect, told a story that Archimedes caught a goldsmith cheating King Hiero II, who suspected that silver had been added to a golden crown. A popular version has him discover buoyancy in a bath and run through the streets shouting "Eureka!", but it was written down long after he lived, in the first century BC, yet some scholars question the method it describes. Galileo doubted the story. In 1586, aged 22, he authored La Bilancetta ("The Little Balance"), a brief treatise that questioned Vitruvius's account of the crown test and proposed a method of his own using water. The overflow method is awkward to carry out: swapping pure gold for a wreath of 1000 grams that is 30 percent silver changes the water level in a 20-centimeter-wide container by only about 0.41 millimeters.
Water can reveal a lumpy object's volume, because an object put under water pushes aside water of its own volume. Weighing an object once in air and again under water shows an apparent loss of weight equal to the displaced water, which at 1 gram per cubic centimeter gives its volume. Dividing mass by that volume gives density, and the same weighing works on a coin. Specific gravity compares an object's density with that of water, so a value under 1 means floating and a value over 1 means sinking. Picture three balls of 10 cubic centimeters each, made from cork, aluminum and lead, with masses 2, 27 and 113 grams; each displaces 10 grams of water and feels the same buoyant force, yet only the cork rises while the aluminum and lead sink. Dividing each mass by 10 cubic centimeters gives densities of 0.2, 2.7 and 11.3 grams per cubic centimeter, and only the cork is below the figure of roughly 1 for fresh water.
Frozen water floats on liquid water because the ice is the less dense of the two. One source gives ice a density about 9 percent under that of liquid water, which leaves an ice cube with most of its volume below the water line. Another gives it as roughly 90 percent of water's density, which works out to about 10 percent less, and adds that the figure can vary, since ice can contain air. In ice, hydrogen bonds link water molecules into open hexagon rings, and the rings leave a lot of empty room. That open arrangement is the reason ice is less dense than the more irregular molecules of liquid water. Another form of the solid behaves differently: heavy water (D2O) frozen into "heavy ice" has about 10.6 percent more density than normal water, which means it sinks in plain water.
Water has a second oddity: at normal air pressure, once ice has melted, the liquid keeps getting denser as it warms until it reaches about 4 °C, and a 2026 preprint, not yet peer reviewed, describes the same behavior. Its peak is 1.00000 grams per cubic centimeter at 39.2°F (4.0°C), compared with 0.99987 at 32°F (0°C) and 0.95865 at 212°F (100°C); by subtraction, water at 0°C is 0.00013 grams per cubic centimeter less dense than at its peak, and water at 100°C is 0.04135 less dense. Water's odd density keeps lakes from freezing solid; suppose water were densest at the freezing point: cold surface water would then sink, a lake might freeze upward from its bottom, and some frozen lakes might not thaw in summer. In real lakes, the densest water sinks beneath the colder water around it.
Dissolved salt raises water's density too. Sea water is slightly denser than fresh water, between 1.02 and 1.03 grams in every cubic centimeter, which explains why a ship rides higher in salty sea water than in fresh water. A line on a ship's hull, called the Plimsoll line, shows how far the ship may safely sink into the water. Shaped as a circle crossed by a horizontal line, the Plimsoll mark is recognised worldwide. Which part of the line the water may reach varies with temperature, saltiness, season and place. In 1870 the British MP Samuel Plimsoll opened an investigation into ship safety. A law of 1876, six years after that investigation began, made load lines compulsory, and the line's exact position was set in law in 1894, eighteen years after that.
The University of Hawaii's figure for the Dead Sea's salinity comes to about 280 parts per thousand, around eight times saltier than average seawater at 35 parts per thousand (as of November 2026), a gap of 245 parts per thousand. Water that salty supports people so firmly that they float with ease, and only a very small number of living things survive in it. The Dead Sea gets very little rain and is hot, so water evaporates and leaves its salts behind; since there is a limit to how much salt dissolves, salt crystallizes out and piles up on the bottom. Britannica adds that the Dead Sea's deep water was saturated with sodium chloride, and being so dense, it stayed on the bottom for a long time. Britannica also reports that the Jordan River's inflow of fresh water was cut sharply after the 1960s. Later, in the late 1970s, the Dead Sea's upper layer was saltier and denser than the layers below it (as of 2026).
A floating hydrometer measures density because the fraction submerged depends on the fluid's density; it is a stem of glass above a weighted bulb, and marks along the stem show where it rides in the liquid. The glass is filled with air and weighted with lead, the hydrometer floats highest in the densest fluids, and it is marked to read specific gravity. Schools use hydrometers to measure the salinity of water, dairies to estimate milk fat (higher fat is less dense), and home brewers to track sugar and fermentation. A hot-air balloon floats because the air heated inside it has lower density than the cooler air outside. When its burners are turned off and the air inside cools, a vent at the top lets cooler outside air in, raising the density inside until the balloon slowly sinks toward the ground. Gas can lift things as water does: a helium balloon is pushed up by air much as a cork is pushed up by water.
An object whose specific gravity is exactly 1 stays suspended in a fluid, neither sinking nor floating, and scuba divers aim for it in order to hover. Even the seemingly solid Earth has fluid characteristics: mountain ranges float on the higher-density crust and mantle beneath them, and lava rises in a volcano for the same reason things float in water. A submarine controls its depth with ballast tanks: letting water in makes it dive, and forcing it out with compressed air lowers its average density below that of the water around it, so it surfaces. Fish manage depth with a gas-filled swim bladder. As a swim bladder expands, the fish gains volume while its mass stays the same, its specific gravity drops, and it moves up; when the bladder shrinks, it moves down.
A shark has no swim bladder; its large oily liver helps it sink more slowly, and its lighter cartilage skeleton and glider-like body and fins help it stay up. Tagging bluntnose sixgill and prickly sharks with motion loggers showed they swam at 200–300 meters at night and below 500 meters by day, and they worked harder on the way down than up, so they are positively buoyant. The tags put the sharks, two kinds in all, at 200–300 meters at night, a band 100 meters wide, and below 500 meters by day, a difference of at least 200 meters. These deep-sea sharks glided uphill for several minutes. The study suggests their positive buoyancy may help them hunt stealthily from below, or help them rise at night when their muscles are coolest.
Marking a ship's safe load is an old idea, older than the laws of the 19th century. As early as the 12th century, ships from Venice carried a cross as a load mark, but load lines became widespread only in the 19th century, after overloaded "coffin ships". Lloyd's Register tried marks in 1835, and they applied only to ships it classed itself. Life jackets give a person lift in the water, and children should wear them, as should everyone aboard a boat. They also come in child sizes, because a jacket made for adults might not suit a child. Most adults, the U.S. Coast Guard reports, can keep their heads above water with only 7 to 12 pounds of buoyancy, which equals 31 to 53 newtons. Ask first, and enter water to play or swim only with a grown-up watching.
Words to know
- hexagon: six-sided shape
- salinity: saltiness
- hydrometer: density meter
- crystallizes: forms crystals
