The weight of a unit volume of a substance is called **specific weight** (\(\gamma\) also known as the unit weight) and is equal to:

\[\gamma =\rho g=\left[\dfrac{\textrm{N}}{\textrm{m}^3}\right]\]

where (rho) is the density and (g) is the gravitational acceleration.

Commonly the term “specific gravity” is improperly used as a synonym for density and for this reason it is very often referred to as g/cm^{3} or kg/L or kg/dm^{3}. In this case grams should be understood according to an obsolete definition of grams weight, not grams mass, where 1 gram weight is the weight of 1 gram mass under standard gravity acceleration.

The difference is subtle and for the truth in practice it can often be ignored, but it is important to keep in mind that while density is a ratio between a mass and a volume, the specific gravity is a ratio between a weight (therefore a force) and a volume. Since the weight is equal to the mass multiplied by the acceleration of gravity expressed in g, the specific gravity (expressed in kg_{weight}/m^{3}) and the density have the same value only if we are in a point where the acceleration of gravity is exactly equal to gn (standard gravity that for convention is equal to 9,80665 m/s^{2} that is 1 g).