Specific weight is defined as the weight per unit volume and is obtained by dividing the weight of a system by the volume it occupies. It is represented by the symbol γ (Greek letter gamma) and in the International System of Units it is measured in N/m3 (newtons per cubic meter).
It is a related term relative density, also called specific gravity, since weight is mass times gravity and density is mass divided by volume. In fact, specific gravity can be calculated as density by gravity:
Where:γ is the specific weight w is the weight of the substance v is the volume m is the mass g is the acceleration due to gravity ρ is the density of the substance
Specific gravity is often used as a property of a fluid and as such it is used in fluid mechanics as a representation of the force exerted by gravity on a volume of a fluid material, hence the units of specific gravity (N/m3) express a force per unit volume. It is also used in soil mechanics, mechanical engineering and many other fields.
Specific gravity in SI
Strictly speaking, specific weight would not fit the International System’s definition of specific property, since a specific property is defined as a property per unit of mass, not volume as is the case with specific gravity.
A specific property is defined as a composite property that is obtained by dividing an extensive property by the mass of the system. Extensive properties are those that change when the total mass of the system changes, but specific properties become intensive properties and remain constant with changes in mass.
For example, volume in the International System is measured in cubic meters (m3) and is an extensive property as it depends on mass. For example, 4 kg of a certain material can occupy 10 m3. The specific volume is obtained by dividing the volume by the mass of the system and represents the volume occupied per unit of mass; in this case 10/4 = 0.25m3/kg, which means that 1 kg of material occupies 0.25 m3. The specific volume, unlike the normal volume, becomes an intensive property, as it is the same as the amount of mass changes: if instead of 4 kg of material we have 8 kg, the system will occupy 20 m3 and 20/8 is also equal to 0.25. The volume changes, the specific volume does not.
Specific gravity, being a property related to volume and not to mass directly, does not enter the definition, although it is a widely used and accepted term.