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The API gravity is an arbitrary scale developed by the American Petroleum Institute (API) as a measure of the density of petroleum liquids at 60 °F relative to the density of water at 60 °F . The values of API gravity are expressed as degrees API denoted as °API.

The API gravity scale has been universally adopted and commonly used in the petroleum industry worldwide since the about 1920.

Relationship between API gravity and specific gravity

The relationship between API gravity and specific gravity (SG) of a petroleum liquid is mathematically expressed as:

$^{\circ }\mathrm {API} ={\frac {141.5}{\mathrm {SG} _{60\,^{\circ }\mathrm {F} }^{60\,^{\circ }\mathrm {F} }}}-\mathrm {131.5}$

and where:

$\mathrm {SG} _{60\,^{\circ }\mathrm {F} }^{60\,^{\circ }\mathrm {F} }={\frac {\mathrm {density\;of\;water\;at\;60\,^{\circ }\mathrm {F} } }{\mathrm {density\;of\;petroleum\;liquid\;at\;60\,^{\circ }\mathrm {F} } }}$

Conversely, the specific gravity may be obtained from the API gravity by this expression:

$\mathrm {SG} _{60\,^{\circ }\mathrm {F} }^{60\,^{\circ }\mathrm {F} }={\frac {141.5}{^{\circ }\mathrm {API} +131.5}}$

Petroleum liquids with an API gravity of more that 10 °API are lighter (i.e., have a lower density) than water and will float on water since petroleum liquids are immiscible in water. Petroleum liquids that are less than 10 °API are heavier (i.e., have a higher density) than water and will sink in water.