Basics of Density, Specific weight, volume, gravity

DENSITY (Kg/m^3 or N•sec²/m^4 )

Density is defined as mass per unit volume . Mass is a property.

Density can be expressed as

ρ = m / V

SPECIFIC VOLUME (m^3/kg or m^4/ N•sec² )

Specific volume on the other hand is the reciprocal of density.

Specific volume = 1/ density

SPECIFIC WEIGHT ( N/m³ or lb/ft³ or kg/m²•sec²)

Specific Weight is defined as weight per unit volume. Weight is a force .

γ = ρ g


For liquids defined as the ratio of the density of a substance to the density of water - at a specified temperature.

SG = ρsubstance / ρH2O

The reference density of water at 4oC (39oF) is used as the reference as these are the conditions of maximum density.

For gases

SG = Mass of gas / Mass of air = Mass of gas / 28.97