The specific gravity of a substance, designated as Gs, is defined as the ratio of the density of that substance to the density of distilled water at a specified temperature. Since it is a ratio, the value of Gs does not depend on the system of units used and is a numerical value having no units. For soil solids, Gs maybe written as:

Gs = density of soil solids / density of water
= mass of soil solids / mass of an equal volume of water

In soil mechanics, the specific gravity of soil solids (which is often referred to as the specific gravity of soil) is an important parameter for calculation of the weight-volume relationship. The specific gravity of soil solids can be determined by the following methods:

1. Density bottle
2. Specific gravity flask
3. Pycnometer

The density bottle is suitable for all types of soil and it is the accurate method whereas the specific gravity flask and pycnometer methods are only suitable for coarse grained soils.