Density is defined as mass per unit volume of a fluid. The density of crude oil and liquid hydrocarbons is usually reported in terms of specific gravity (SG) or relative density, defined as the density of the liquid material at 60°F (15.6°C) divided by the density of liquid water at 60°F. At a reference temperature of 15.6°C, the density of liquid water is 0.999 g/cm3 (999 kg/m3), which is equivalent to 8.337 lb/gal (U.S.). Therefore, for a hydrocarbon or a petroleum fraction, the SG is defined as:
In the early years of the petroleum industry, the American Petroleum Institute (API) adopted the API gravity (°API) as a measure of the crude oil density. The API gravity is calculated from the following equation:
The API scale for gravity was adapted from the Baumé scale, developed in late 18th century to be used in hydrometers for measuring even small differences in the specific gravity of liquids, using water as a reference material in these devices. A liquid with SG of 1 (i.e., water) has an API gravity of 10. One can note from Eq. 1 that liquid hydrocarbons with lower SGs have higher API gravities. The API of crude oils varies typically between 10 and 50, with most crude oils falling in the range of 20-45. Using API gravity, the conventional crude oils can be generally considered as light (°API>30), medium (30>°API>22), and heavy (°API<22).
Note that the relationship between °API and specific gravity is not linear. Therefore, the °API gravity of crude blends cannot be calculated by linear averaging of the component °APIs. Specific gravities of the components can be averaged, though, to determine the specific gravity of the resulting blend. In practice, averaging °APIs is usually accepted because the error involved in averaging is small.
Among the hydrocarbons, aromatic hydrocarbons have higher SG (lower °API) than paraffinic hydrocarbons with the same number of carbon atoms. For example, benzene has an SG of 0.883 (°API of 28.7), whereas n-hexane has an SG of 0.665 (°API of 81.3). Therefore, the heavy (high-density) crude oils tend to have high concentrations of aromatic hydrocarbons, whereas the light (low-density) crude oils have high concentrations of paraffinic hydrocarbons.