API Gravity Calculator
This calculator converts between specific gravity and API gravity, the scale the American Petroleum Institute uses to describe how light or heavy a petroleum liquid is compared with water. Choose your conversion direction, then enter either the specific gravity of your sample (relative to water at 60 degrees Fahrenheit) or its API gravity in degrees, or pick a preset such as light crude, medium crude, diesel or petrol to fill the specific gravity field for you. The calculator applies the API formula, API Gravity = (141.5 / SG) - 131.5, or its reverse, and returns the API gravity, the specific gravity, the density in kilograms per cubic metre at 15.6 degrees Celsius, and a classification of light, medium, heavy or extra heavy crude oil with the matching API range. A breakdown panel shows the formula and working, plus whether the liquid floats or sinks, a typical example for that grade, and how relative value and refining complexity change with density. Use it to check where a crude sample sits on the API scale, convert lab readings between the two units, or see how density affects petroleum pricing and refining. Because both measurements assume the standard 60 degree Fahrenheit reference, treat results as indicative only if your sample was measured at a different temperature, since a correction factor would then be needed for laboratory-accurate figures.
1. Conversion Direction
2. Petroleum Sample (optional)
API gravity greater than 10 means the liquid floats on water. API gravity less than 10 means it sinks.
Calculation Breakdown
Classification
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.