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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 (15.56 °C) . 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:

$\mathrm {SG} _{60\,^{\circ }\mathrm {F} }^{60\,^{\circ }\mathrm {F} }={\frac {\mathrm {density\;of\;water\;at\;60\,^{\circ }\mathrm {F} } }{\mathrm {density\;of\;petroleum\;liquid\;at\;60\,^{\circ }\mathrm {F} } }}={\frac {\mathrm {density\;of\;water\;at\;15.56\,^{\circ }\mathrm {C} } }{\mathrm {density\;of\;petroleum\;liquid\;at\;15.56\,^{\circ }\mathrm {C} } }}$

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.

Measurement of API gravity

API gravity can be measured using a hydrometer graduated with the API scale as per ASTM D-287, Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method).

Alternatively, the density of the petroleum liquid may be measured at 60 °F may be determined and divided by the density of water at 60 °F to obtain the specific gravity of the liquid at 60 °F, which can then be converted to the °API using the equation in the previous section above.