What is a relative density

relativ density

The relativ density d, also specific density, describes the relationship between two densities.

  • the ratio of the density ρ of a substance to the density ρ0 one Normal condition of this substance
  • the density ρ of a substance to the reference density ρ0 one Upholstery fabric
$ d = {\ rho \ over \ rho_0} $

As a size ratio, the relative density is a dimensionless quantity without a unit. The density itself is Mass per volume (Unit: kg / m³). For definition see DIN 1306.

Reference values

Common standard reference densities (Normal) are the density of pure water under normal conditions at 3.98 ° C and the density of dry air under normal conditions (0 ° C and 1013.25 mbar = 1.2931 kg / m³) or standard conditions (20 ° C or 25 ° C at same air pressure).

The relative density $ d ^ {T1} _ {T2} $ is the ratio of the mass of a certain volume of a liquid at temperature T1 to the mass of the same volume of water at temperature T2. A common relative density is $ d ^ {20} _ {20} $, which describes the density of a liquid at 20 ° C in (e.g. 2 g / cm³) in relation to the density of water at 20 ° C (approx g / cm³). The relative density would then be 2, so the liquid, at 20 ° C, is twice as dense as water at 20 ° C. Another example is $ d ^ {20} _ {4} $, this describes the density of a liquid at 20 ° C in relation to the density of water at 4 ° C.[1]


In mineralogy and engineering, density information is mostly related to the density of water. The use is useful when a material is e.g. B. as a result of different temperatures in different structures, e.g. B. is present with greater or lesser porosity.

In the case of gases, it is mostly used to show the "weight ratio" to air. See DIN 1871 (May 1999), in which the relative density of a gas is defined as the quotient of the density of a gas and the density of dry air at the same pressure and temperature.

A gas with a relative density <1.0 is lighter than air, i.e. it rises, and a gas with a relative density> 1.0 is heavier than air and therefore falls: natural gas, for example, has a relative density of 0.55 up to 0.75 and liquid gas or propane from 1.55.


  • DIN 1306 Density; Terms, information
  • DIN 1871 Gaseous fuels and other gases - Density and other volumetric quantities
  • Bergmann, Schaefer: Experimental Physics Textbook, Volume 1, 11th edition

Individual evidence

  1. ↑ The International pharmacopoeia, Volume 1 in the Google book search