Water is one of the most common substances on the planet, as well as being vital to life. As a result, the physical properties of water have been well studied and are known to a high degree of precision. The unit weight of water is one such property. It can be expressed in a variety of ways, depending on the particular units chosen.
Weight vs. Mass
Most people don't make a distinction between mass and weight, but to a scientist, these are two different but related concepts. Mass is the actual physical bulk of a substance. Weight is the force with which gravity pulls on that mass. For example, a cat has more mass than a mouse, but a cat on the moon might weigh less than a mouse on Earth. For this reason, the units used to describe a unit weight of water sometimes represent the force of gravity.
Concept of Density
The unit weight of water is essentially the same as the density of water. Density is a measure of how much mass a substance has per unit volume. A small volume of a very dense material like lead will be heavy, whereas a large volume of a low-density material, such as Styrofoam, can weigh little. Density is normally given in units of mass per volume; for example, grams per cubic centimeter.
Unit Weight of Water
Scientists and engineers use two main measurement systems to define the unit weight of water. In the metric, or SI, system the unit weight of water is 998 kg per cubic meter -- kg/m3 -- or approximately 1 g per cubic centimeter, g/cm3. In imperial measurements, the unit weight of water is commonly given as 62.4 pounds per cubic foot, lbs/ft3. A further way to describe unit weight using the metric system and referencing the force of gravity is 9,800 newtons per cubic meter, N/m3.
Variations in Unit Weight
The density of any substance will change with pressure and temperature as the material expands and contracts, and water is no exception. The unit weight of water given by reference sources is typically the weight at standard atmospheric pressure and at or around room temperature. The density of water reaches a maximum at about 4 degrees Celsius, although the difference in density between this temperature and room temperature is only around 1 percent. Increasing the pressure to several hundred times normal atmospheric pressure will also only produce an approximate 1 percent increase in unit weight.