Physics turns the push of water into one short rule. Water pressure grows with depth, and the difference between the pushes on the bottom and top of an object is the buoyant force. In symbols, Archimedes' principle reads F_B = w_fl, which says the buoyant force equals the weight of the fluid displaced. The rule covers every object and every fluid, whether the object is partly or wholly submerged. A floating object settles until the water it displaces weighs exactly what the object weighs. A newly launched ship sinks into the sea to exactly that depth.

The rule predicts results that can be tested, and here is one that you can picture. Objects of equal volume feel equal buoyant forces. Take balls of cork, aluminum and lead of 10 cubic centimeters, which have masses of 2, 27 and 113 grams, yet each one displaces 10 grams of water. The cork rises and the other two sink. Specific gravity gives the same answer in one number. It compares an object's density with the density of water, and under 1 means it floats while over 1 means it sinks. The three balls take up the same amount of space, so only their masses differ.

A hydrometer turns floating into a measurement. Picture it bobbing in a tall jar of liquid. It is a glass stem with a weighted bulb. Lines etched on the stem mark how high or low it floats, and that reading gives the liquid's relative density, or specific gravity. A hydrometer in fresh water, which has a density of about 1 gram per cubic centimeter as of November 2026, can be compared with one in sea water, which is about 1.02 to 1.03 grams per cubic centimeter. The hydrometer floats lower in a less dense liquid and higher in a denser one.

Ice is less dense than liquid water, which is why it floats. Because ice is about 9 percent less dense than liquid water, most of a floating ice cube lies below the water line. Cooking oil floats on top of water as well. One kind of ice breaks the pattern. Ice made from heavy water (D2O), called "heavy ice", is about 10.6 percent denser than normal water, so it sinks in ordinary water. Ice floats on water, and a lump of clay or iron sinks. Ice, oil and heavy ice each show what density can do.

Water as salty as the Dead Sea holds people up so strongly that floating is easy. Now look at the numbers, which show how salty it is. According to the University of Hawaii, as of November 2026, the salinity of the Dead Sea is about 280 parts per thousand. Average seawater is 35 parts per thousand, so the Dead Sea is around eight times saltier. Bathers stay buoyant in the Dead Sea because it is so salty and dense, and very few living things can survive there. Salt water has a higher density than fresh water.

Look at ships next, and at how low they sit. The Plimsoll line, painted on a ship's side, shows how low the ship may safely sit in the water. British MP Samuel Plimsoll began to investigate ship safety in 1870. Load lines became compulsory by a law in 1876, and a law in 1894 fixed the line's position. Sea water is denser, so a ship sits higher there than it would in fresh water. An empty ship sits higher in the water than the same ship loaded with cargo.

Vehicles can change their own density. Air heated inside a hot-air balloon is less dense than the cooler air outside, and that is why the balloon floats. For a descent, the heat goes off and a vent at the top admits cooler air. A submarine goes down by flooding its ballast tanks with water. It surfaces when compressed air forces water out of its ballast tanks, which lowers its average density below the density of the water around it. Gases give buoyancy too: a helium balloon is lifted by the air just as a floating cork is lifted by water.

Everything in water is pushed upward. Fish and sharks show how living things work with that push. Gas fills the swim bladder that most fish have, and it helps them hang in the water. If a fish's swim bladder expands, its volume grows but its mass stays the same, so its specific gravity falls and the fish moves up. A shrinking bladder moves the fish down. Sharks have no swim bladder, but a big oily liver helps them sink more slowly. A swim bladder lets a fish hang in the water, and scuba divers aim for the same state: an object with the same density as the fluid hangs in it, neither sinking nor rising.

Recall that density is the mass packed into a certain volume. It helps to think of a whole lake at once. Water changes density as it cools, because cold water is generally denser than warm water. At about 39°F (4°C) water is at its densest, so in a lake the densest water sinks beneath colder water. As a result, lakes do not freeze from the bottom up, and ice floats on water too. Ice is less dense because its molecules of water are arranged in a spread-out pattern, with plenty of room between them.

Depth does not change whether something floats: something that floats in 2 feet of water will also float in 2000 feet of water. According to the U.S. Coast Guard, most adults need only 7 to 12 pounds (31 to 53 newtons) of buoyancy to keep their heads above water. Life jackets help people float. Children and everyone aboard a boat should wear one. A jacket made for adults may not work for a child, so life jackets come in child sizes. Ask first, and play or swim only when a grown-up is watching you.

Words to know

• submerged: under water
• salinity: saltiness
• compulsory: required
• relative: compared
• bladder: gas sac