API Gravity Equations and Calculator

Related Resources: fluid flow

API Gravity Equations and Calculator

Fluids Engineering

API Gravity or American Petroleum Gravity Equations and Calculator

API Gravity is the quantity of heavy or light a petroleum fluid is relative to water.

Eq. 1
API = 141.5 / SG_o - 131.5

Eq. 2
SG_o = 141.5 / ( API + 131.5 )

Eq. 3
SG_o = ρ_oil / ρ_water at 60 F

Where:

SG_o = Specific Gravity of Oil Phase (fraction)
API = API Gravity (dimensionless)
ρ_oil = density of petroleum fluid
ρ_water = density of water

The official density of water at 60 °F according to the 2008 edition of ASTM D1250 is 999.016 kg/m³ . The 1980 value is 999.012 kg/m³ . In some cases the standard conditions may be 15 °C (59 °F) and not 60 °F (15.56 °C), in which case a different value for the water density would be appropriate

Generally speaking, oil with an API gravity between 40 and 45° commands the highest prices. Above 45°, the molecular chains become shorter and less valuable to refineries.

Crude oil is classified as light, medium, or heavy according to its measured API gravity.

Light crude oil has an API gravity higher than 31.1° (i.e., less than 870 kg/m³ )
Medium oil has an API gravity between 22.3 and 31.1° (i.e., 870 to 920 kg/m³ )
Heavy crude oil has an API gravity below 22.3° (i.e., 920 to 1000 kg/m³ )
Extra heavy oil has an API gravity below 10.0° (i.e., greater than 1000 kg/m³ )

Source

Report on the Development, Construction, Calculation and Preparation of the ASTM – IP Petroleum Measurement Tables (ASTM D1250; IP 200), 1960, published by the Institute of Petroleum, 61 New Cavendish Street, London, W.1 and American Society for Testing Materials, 1916 Race Street, Philadelphia 3, Pa.
API Manual of Petroleum Measurement Standards, Chapter 11.1 – 1980,Volume XI/XII, Adjunct to: ASTM D1250-80 and IP 200/80
"Comments on Crude Oil Gravity Adjustments". Louisiana Department of Natural Resources. Archived from the original on 2 May 2006. Retrieved 6 January 2014.
USGS FS2006-3133_508, web PDF file: USGS-508-PDF