API gravity
Encyclopedia
The American Petroleum Institute
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...

gravity, or API gravity, is a measure of how heavy or light a petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API gravity is thus an inverse measure of the relative density of a petroleum liquid and the density of water, but it is used to compare the relative densities of petroleum liquids. For example, if one petroleum liquid floats on another and is therefore less dense, it has a greater API gravity. Although mathematically, API gravity has no units (see the formula below), it is nevertheless referred to as being in "degrees". API gravity is gradated in degrees on a hydrometer instrument. The API scale was designed so that most values would fall between 10 and 70 API gravity degrees.

## History of development

The U.S. National Bureau of Standards in 1916 established the Baumé scale (see: Degree Baumé) as the standard for measuring specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

of liquids less dense than water (see: Density of water). Investigation by the U.S. National Academy of Sciences found major errors in salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

and temperature controls that had caused serious variations in published values. Hydrometers in the U.S. had been manufactured and distributed widely with a modulus of 141.5 instead of the Baumé scale modulus of 140. The scale was so firmly established that by 1921 the remedy implemented by the American Petroleum Institute
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...

was to create the API Gravity scale, recognizing the scale that was actually being used.

## API gravity formulas

The formula to obtain API gravity of petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

liquids, from specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

(SG), is:

Conversely, the specific gravity of petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

liquids can be derived from the API gravity value as

Thus, a heavy oil with a specific gravity of 1.0 (i.e., with the same density as pure water at 60°F) would have an API gravity of:

## Using API gravity to calculate barrels of crude oil per metric ton

In the oil industry, quantities of crude oil are often measured in metric tons. One can calculate the approximate number of barrels per metric ton for a given crude oil based on its API gravity:

So, for example, a metric ton of West Texas Intermediate
West Texas Intermediate
West Texas Intermediate , also known as Texas light sweet, is a grade of crude oil used as a benchmark in oil pricing. It is a light and sweet crude oil...

(39.6° API) would contain about 7.6 barrels.

## Measurement of API gravity from its density

To derive the API gravity from the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

, the density is first measured using either the hydrometer
Hydrometer
A hydrometer is an instrument used to measure the specific gravity of liquids; that is, the ratio of the density of the liquid to the density of water....

, detailed in ASTM D1298 or with the oscillating U-tube
Oscillating U-tube
The oscillating U-tube is a technique to determine the density of liquids and gases based on an electronic measurement of the frequency of oscillation, from which the density value is calculated. This measuring principle is based on the Mass-Spring Model....

method detailed in ASTM D4052.
Density adjustments at different temperatures, corrections for soda-lime glass expansion and contraction and meniscus
Meniscus
The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the material of the...

corrections for opaque oils are detailed in the Petroleum Measurement Tables, details of usage specified in ASTM D1250.
The specific gravity is then calculated from the formula below and the API gravity calculated from the first formula above.

## Direct Measurement of API gravity (Hydrometer method)

Thves the advantages of field testing and on-board conversion of measured volumes to volume correction.
This method is detailed in ASTM D287.

Generally speaking, oil with an API gravity between 40 and 45 commands the highest prices. Above 45 degrees 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 is defined as having an API gravity higher than 31.1 °API. (less than 870 kg/m3)
• Medium oil is defined as having an API gravity between 22.3 °API and 31.1 °API. (870 to 920 kg/m3)
• Heavy oil is defined as having an API gravity below 22.3 °API. (920 to 1000 kg/m3)
• Extra heavy oil is defined with API gravity below 10.0 °API. (greater than 1000 kg/m3)

Not all parties use the same grading. The United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...

uses slightly different definitions.

Crude oil with API gravity less than 10 °API is referred to as extra heavy oil or bitumen. Bitumen derived from the oil sands deposits in the Alberta, Canada area has an API gravity of around 8 °API. It is 'upgraded' to an API gravity of 31 °API to 33 °API, and the upgraded oil is known as synthetic crude
Synthetic crude
Synthetic crude is the output from a bitumen/extra heavy oil upgrader facility used in connection with oil sand production. It may also refer to shale oil, an output from an oil shale pyrolysis. The properties of the synthetic crude depend on the processes used in the upgrading. Typically, it is...

.