What is API gravity and how is it calculated?

API gravity is a measure of how heavy or light petroleum is relative to water, expressed in degrees (°API). It is defined by the American Petroleum Institute using the formula: API Gravity = (141.5 / SG) - 131.5, where SG is the specific gravity of the liquid measured at 60°F (15.6°C) relative to water at the same temperature. A liquid with an API gravity greater than 10 is lighter than water and will float; a liquid with an API gravity less than 10 is heavier than water and will sink. Most crude oils range from about 10 to 70 degrees API.

What do the API gravity classification grades mean?

The petroleum industry classifies crude oil by API gravity into four grades. Light crude oil has an API gravity above 31.1 degrees and flows easily, commands a higher price, and yields more petrol and diesel during refining. Medium crude oil falls between 22.3 and 31.1 degrees API. Heavy crude oil is between 10.0 and 22.3 degrees and requires more energy-intensive refining. Extra heavy crude (below 10 degrees API) is denser than water and includes bitumen and tar sands. Light crudes are generally more valuable because they require less processing to produce high-value products.

How do I convert API gravity back to specific gravity?

To convert API gravity back to specific gravity, rearrange the API formula: Specific Gravity = 141.5 / (API + 131.5). For example, an oil with 35 degrees API has a specific gravity of 141.5 / (35 + 131.5) = 141.5 / 166.5 = 0.8499, meaning it is about 85% the density of water at 60°F. Both measurements reference 60°F (15.6°C), which is the standard temperature for petroleum density measurements.

API Gravity Calculator

Convert specific gravity to API degrees or API gravity back to specific gravity. Classify petroleum fluids as light, medium, heavy, or extra heavy crude oil based on the American Petroleum Institute (API) gravity scale.

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Standard formula American Petroleum Institute definition. All measurements reference 60°F (15.6°C).

1. Conversion Direction

Calculate

Specific Gravity (relative to water at 60°F)

API Gravity (degrees °API)

°API

2. Petroleum Sample (optional)

Select a common petroleum reference (fills SG above)

API gravity greater than 10 means the liquid floats on water. API gravity less than 10 means it sinks.

API Gravity Result

API Gravity
-
°API

Specific Gravity

at 60°F vs water

Density

kg/m³ at 15.6°C

Classification

Calculation Breakdown

Input value-

Formula used-

Numerator / divisor-

API Gravity-

Specific Gravity-

Density (kg/m³)-

Classification

Grade-

API range-

Floats on water?-

Typical example-

Relative value-

Refining complexity-

Result: Enter a specific gravity or API value above.

Worked example (default values): A light crude oil has a specific gravity of 0.85 at 60°F. API Gravity = (141.5 / 0.85) - 131.5 = 166.471 - 131.5 = 34.97 °API. Density = 0.85 × 999.016 = 849.16 kg/m³. This classifies as light crude oil (above 31.1 °API).

What is API Gravity?

API gravity is a measure developed by the American Petroleum Institute (API) to compare the density of petroleum liquids to water. The scale was designed so that most petroleum products fall in a convenient numeric range. Water has an API gravity of exactly 10 degrees. Liquids lighter than water (most crude oils, petrol, diesel) have API gravity above 10 and float on water. Liquids heavier than water (bitumen, some heavy crude oils) have API gravity below 10 and sink.

The scale is widely used in the petroleum industry for pricing, classification, refining decisions, and pipeline specifications. Higher API gravity generally indicates a lighter, more valuable crude oil that yields more high-value products such as petrol and jet fuel during refining.

The API Gravity Formula

API gravity is calculated from specific gravity (SG) measured at 60°F (15.6°C):

API Gravity (°API) = (141.5 / SG) - 131.5

To convert API gravity back to specific gravity:

SG = 141.5 / (API + 131.5)

Specific gravity itself is the ratio of the fluid's density to the density of water at 60°F (approximately 999.016 kg/m³). To find actual density in kg/m³, multiply specific gravity by 999.016.

The constants 141.5 and 131.5 were chosen empirically by the API and NIST (National Bureau of Standards) in the early twentieth century. They place water at exactly 10 degrees API and ensure that most petroleum products fall in the range of 10 to 70 degrees API.

API Gravity Classification

Grade	API Gravity Range	Specific Gravity Range	Typical Examples
Light crude	Above 31.1 °API	Below 0.870	West Texas Intermediate (WTI), North Sea Brent
Medium crude	22.3 to 31.1 °API	0.870 to 0.920	Many Middle Eastern crudes
Heavy crude	10.0 to 22.3 °API	0.920 to 1.000	Venezuelan heavy crude, some Canadian blends
Extra heavy crude	Below 10.0 °API	Above 1.000	Bitumen, Orinoco tar sands, Athabasca oil sands

Why API Gravity Matters

Light crude oil (high API gravity) is easier to transport through pipelines, requires less energy-intensive processing in a refinery, and yields a higher proportion of valuable light products such as petrol, jet fuel, and kerosene. For these reasons, light crude oils trade at a premium over heavy grades on international markets.

Heavy crude oils require more sophisticated refining equipment (such as hydrocracking and cokers) to convert heavy fractions into lighter, more saleable products. Extra heavy crude and bitumen may need to be diluted with lighter oils (diluent) just to flow through a pipeline.

API gravity is not the only factor in crude oil valuation. Sulphur content (sweet vs sour), metal content, and viscosity also affect price. However, API gravity is typically the primary density measure used in contracts, pricing benchmarks, and refinery planning.

Temperature Reference

Both specific gravity and API gravity measurements must be taken at the reference temperature of 60°F (15.556°C, approximately 15.6°C). Petroleum liquids expand when heated and contract when cooled, so measuring at different temperatures produces different values. If a sample is measured at a different temperature, a correction factor (using the ASTM volume correction factor tables or the API Manual of Petroleum Measurement Standards) must be applied before using the API formula.

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Method and sources: Formula from the American Petroleum Institute (API) and ASTM Standard D1298 (Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method). Classification grades per API and the Society of Petroleum Engineers. Reference water density at 60°F from NIST/IAPWS.

This calculator applies the standard API gravity formula at the reference temperature of 60°F (15.6°C). For accurate laboratory results, temperature correction using ASTM tables is required if the sample is not at 60°F. API gravity is defined for petroleum liquids and may not apply to non-petroleum fluids.