API Gravity to Specific Gravity (Crude Oil and Condensate)

The API scale runs the opposite way to density. A higher API gravity means a lighter oil, and water sits at 10 degrees API, so anything above 10 floats on it. A hydrometer reads that scale directly (ASTM D287 is the method), but a density calculation wants the specific gravity: the density of the oil divided by the density of water, both at 60 degrees Fahrenheit (15.6 degrees Celsius). That is what this formula gives. The numbers 141.5 and 131.5 belong to the definition of the scale, which the EIA glossary prints in the form $\mathrm{API}=141.5/SG-131.5$.

The page's examples run in both directions. A heavy oil with a specific gravity of 1.0 is 10.0 °API, the same as water. West Texas Intermediate at 39.6 °API gives $141.5/171.1=0.827002$, the 0.827 on the page, and 31.1 °API gives $141.5/162.6=0.870234$. The other way round, a specific gravity of 0.85 is $141.5/0.85-131.5=34.9706$ °API. Notice that the specific gravity falls as the API number rises: the larger $131.5+\mathrm{API}$ in the denominator is the lighter oil.

A specific gravity is a ratio, so for a density multiply it by the density of water at 60 °F, which is 999.016 kg/m³ in the current ASTM tables; that gives the oil's density at that temperature. A lighter oil takes more volume for the same mass: a tonne of West Texas Intermediate fills 7.61 barrels, which agrees with Wikipedia's 'about 7.6'.

The temperature is the catch. The scale is defined at 60 °F, so a density measured at another temperature has to be brought back to 60 °F before it is converted; this page does the conversion, not that correction. For the reverse direction, a hydrometer or density meter reading expressed as a specific gravity goes in and the API gravity comes out. The page is for crude oil and condensate. Its sources are the EIA glossary entry for API gravity and ASTM D287, the hydrometer method for API gravity.