Density is the number of things—which could be people, animals, plants, or objects—in a certain area. To calculate density, you divide the number of objects by the measurement of the area.

The population density of a country is the number of people in that country divided by the area in square kilometers or miles. The country of Singapore is one of the most densely populated countries in the world. It has a population of 4,839,400 and an area of 687 square kilometers (265 square miles), so its density is 7,044 people per square kilometer (18,262 per square mile).

Of course, not all the people in the country are spread out evenly. Cities have a greater population density than rural areas.

Density can also be used to refer to the number of plants or animals in a certain area. Sometimes, animal or plant populations are too dense. This leads to overpopulation.

Deer have overpopulated areas of the Midwestern United States, for example. A common reason is a lower density of their natural predators, such as pumas or bears. As a result of the deer’s dense population, the competition for land and food is heightened and many deer die of starvation. Vegetation does not have time to develop, so food becomes scarce. A habitat can only support a limited number of each type of organism, so an overpopulation of deer can drive those deer to other habitats, including human habitats such as towns.

Density can refer to the number of molecules in a substance. This can apply to gases, liquids, and solids.

Air quality is defined by the number of pollutants in a certain area. Air quality therefore measures the density of air pollutants, such as smoke and emissions. Densely populated cities often have poor air quality because of the air pollution density.

Density is used to measure the salinity of seawater. Water densely packed with salt has a high salinity. Water that has few salt molecules has low salinity.

Rocks and minerals are also measured for density. Many igneous rocks, or rocks formed from volcanic eruptions, have a low density. They are full of pockets of air. One type of igneous rock, pumice, has such a low density that it can actually float on water. The water is more dense than the rock.