Objects that float on water, such as ice, ethanol and benzene, are less dense than water. What floats on water also depends on whether the water is fresh or saltwater. Saltwater is more dense than fresh water, and it has a density of 1,030 kilograms per cubic meter. Fresh water has a density of 1,000 kg/m3 at 4 degrees Celsius or 39 degrees Fahrenheit.

Oxygen and helium can float on water, but as gases their density is very dependent on temperature. The density of solid objects is determined by their mass divided by their volume. The formulas for a regular object's volume depend on whether it's a sphere, a rectangular box, a cube or a cylinder. An irregular object's volume is measured by the amount of water it displaces. If a solid has a volume of 30 m3 and a mass of 20 kg, it will have a density of 0.66 kg/m3. Therefore, it floats on water.

So, the size of the object doesn't matter when it comes to determining whether or not it can float on water. Some astrophysicists note that if a tub full of water was made large enough, the planet Saturn, a gas giant, could float in it. Saturn's density is 0.000687 kg/cm3.