A soil particle has no pore space, and is nothing more than a very small piece of rock. The weight of an individual soil particle per unit volume is called particle density. Usually, particle density is expressed in units of grams per cubic centimeter (g/cm3). An average value for particle density is 2.66 g/cm3. This means that a soil particle that is 1 cubic centimeter in volume weighs 2.66 g. In comparison, water has a density of 1 g/cm3, and organic matter has a density of 0.8 g/cm3. In English units, water has a value of 62.4 lbs/ft3. Traditionally, most measurements of soil physical properties are expressed in the metric system. For your reference, 2.54 cm = 1 inch; 454 g = 1 pound.
Particle density plays an important role in our understanding and determination of other physical properties including bulk density and porosity.