API Gravity Calculator

The API gravity is a measure of how much lighter or heavier crude oil is than water. It's one of the first numbers considered by the oil industry and allows for crude oils to be grouped into different quality classes. It also has a direct effect on the price per barrel of crude oil. If you enter in the API gravity, density, or measured specific gravity this tool will calculate the other two values as well as the grade of crude oil, weight in pounds per gallon, and number of barrels per ton.

The importance of API density.

American Petroleum Institute (API) gravity is a measure of density that is inversely stated. Lighter oils have higher API gravities while heavier oils have lower values. This measurement uses water as the reference with an API gravity of 10. So, oil with an API gravity greater than 10 will float while oil with an API gravity less than 10 will sink.

Since the density of the API and the opposite direction vary, a higher API value means that crude oil is lighter in weight and has a higher value. So traders immediately think of light, easily processable crude oil when they hear the term "40 API".

Formula:

The API gravity is calculated from the density of crude oil measured at 60 degrees Fahrenheit.

By rearranging the equation, density can be calculated back from the API value.

The specific gravity (SG) is the ratio of the density of crude oil to that of water at a temperature of 60 °F. Two constants, 141.5 and 131.5, determine the API value for water exactly as 10. To convert the SG into absolute values, it is multiplied by the density of water at 60 °F. According to ASTM D1250, this density is 999.016 kg/m3.

Example calculation:

Assume a lab report indicates a specific gravity of 0.85. Plugging that into the equation yields the following:

Thus this crude oil has an API of about 35 and is classified as a light crude oil. The density would be .85 times 999.016 which comes to approximately 849 kilograms per cubic meter. Converted, that is about 7.08 pounds per gallon or about 7.4 barrels per tonne.

Interpretation of crude oil quality.

Refinery operations and reporting agencies classify crude oil into various ranges based on its API gravity. The following limits correspond to the industry practices commonly used by the API, USGS, and EIA.

The API gravity of condensates and very light oils is over 45. The ideal range for refining is about 40 to 45 API. Crude oil in this range generally trades at the highest prices, as it can provide a relatively balanced share of gasoline and diesel fuel per barrel.

Why 60 degrees F is used as a reference point:

Because density is a function of temperature, API gravity values are only meaningful at one particular temperature. The industry has established that reference temperature to be 60 °F. Measurements made at other temperatures must be converted to 60 °F using a volume correction factor. These factors are documented in the API MPMS Part 11.1 and can then be used for trading purposes.

This calculator assumes that the values entered are already at 60°F. If you have measured values at a different temperature, then you must correct them first and enter the corrected values here.

Areas of application

The API gravity can be used at all stages of crude oil purchase, sale, transportation and refining. Traders use it to grade goods and set prices with reference to benchmark crude oils like WTI or Brent. Pipeline operators and tanker owners use it to calculate how heavy a given volume of crude is. Refineries use it to predict the ratio in which different products can be extracted from a particular crude oil.

This calculator is for learning and quick estimates only. For actual delivery, invoicing or legally binding measurements use the correction procedures and reference tables set out in current ASTM D1250 and API MPMS standards.