Fuel for flight is often calculated as a weight (e.g in lbs or Kgs). However, as the supliers dispense fuel as a volume, it is necessary to convert the fuel weight into a volume. The mass of the fuel is not affected by temperature or pressure. The volume of the fuel, however, is dependent on temperature, pressure and density (ρ) hence we cannot simply use a conversion coefficient. Instead we use the Specific Gravity, which is a measure of the relative density of one substance (i.e. the fuel) to the density of water at a given temperature.
Specific Gravity is a dimensionless unit, as the same units in the fraction cancel each other out. When converting a volume of fuel to a weight of fuel, we multiply using the Specific Gravity, whereas when converting from a weight of fuel to a volume, we divide using the Specific Gravity.