API Gravity Calculator
Convert specific gravity to API gravity for petroleum.
API gravity rates how heavy or light a petroleum liquid is compared to water.
How the Math Works
The API Gravity Calculator uses a specific mathematical transformation to convert a liquid's specific gravity (SG) into degrees American Petroleum Institute (API) gravity. The formula applied is API = 141.5 ÷ SG - 131.5, where 141.5 and 131.5 are constants derived from empirical observations of petroleum properties. This equation inverts the relationship between specific gravity and API gravity, where a higher specific gravity (denser fluid) results in a lower API gravity value, and vice versa. For example, a fluid with a specific gravity of 0.9 would yield an API gravity of approximately 14.8 degrees, calculated by dividing 141.5 by 0.9 and subtracting 131.5.
Practical Applications
To use this calculator in practical scenarios, first measure or obtain the specific gravity of the petroleum product by comparing its density to that of water at a specified temperature, typically 60°F (15.56°C). Input this specific gravity value into the formula or calculator interface. The resulting API gravity is crucial for classifying crude oil grades in the petroleum industry; lighter crude oils (like those with API gravities above 31.1) are more valuable due to their higher refinery yields, while heavier crudes (below 10 API) require more processing. Refineries and traders rely on this calculation to determine pricing, blending strategies, and processing requirements for different petroleum streams.
Day-to-Day Use
While most people may not calculate API gravity daily, this measurement indirectly influences everyday experiences like fuel prices at the pump. The API gravity of crude oil affects how efficiently it can be refined into gasoline, diesel, and other petroleum products that power vehicles, heat homes, and generate electricity. Higher API gravity oils yield more valuable refined products, which can impact regional fuel costs and availability. Additionally, industries beyond petroleum — such as food production and manufacturing — use similar density-based measurements for quality control, making understanding of these concepts valuable for professionals in various scientific and industrial fields.
Worked example
SG 0.85 → about 34.97 °API.
FAQ
What is 'light' crude?
Higher API (lower density) oils are called light; they generally fetch more.