The specific weight, also known as the unit weight (symbol , the Greek letter gamma), is a volume-specific quantity defined as the weight divided by the volume of a material:
Equivalently, it may also be formulated as the product of density, , and gravity acceleration, :
Its unit of measurement in the International System of Units (SI) is the newton per cubic metre (N/m<sup>3</sup>), expressed in terms of base units as kgâ m<sup>âÂÂ2</sup>â s<sup>âÂÂ2</sup>. A commonly used value is the specific weight of water on Earth at , which is .
The density of a material is defined as mass divided by volume, typically expressed with the unit kg/m<sup>3</sup>. Unlike density, specific weight is not a fixed property of a material, as it depends on the value of the gravitational acceleration, which varies with location (e.g., Earth's gravity). In practice, the standard gravity (a constant) is often assumed, usually taken as .
Pressure may also affect values, depending upon the bulk modulus of the material, but generally, at moderate pressures, has a less significant effect than the other factors.
In fluid mechanics, specific weight represents the force exerted by gravity on a unit volume of a fluid. For this reason, units are expressed as force per unit volume (e.g., N/m<sup>3</sup> or lbf/ft<sup>3</sup>). Specific weight can be used as a characteristic property of a fluid.
Specific weight is often used as a property of soil to solve earthwork problems.
In soil mechanics, specific weight may refer to:
Specific weight can be used in civil engineering and mechanical engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation.