API gravity is a measure on how heavy or light petroleum liquid is compared to water. API gravity is a measure of the relative density of petroleum liquid and the density of water. This measurement is used for petroleum such as diesel and bio diesel by manufactures and people making there own bio diesel at home. This measurement is basically used for all petroleum products at hand. If you would
. like further information on this measurement, click the link provided for you here. http://www.engineeringtoolbox.com/api-gravity-d_1212.html