Table of Contents
What Is API Gravity?
API gravity is an inverse measure of petroleum liquid density relative to water, established by the American Petroleum Institute. Water has an API gravity of 10. Lighter oils have higher API values (gasoline is about 60 API), while heavier oils have lower values (heavy crude is around 20 API). The scale was developed because lighter, less dense oils are generally more valuable and easier to refine.
API gravity is the standard density measure used throughout the petroleum industry for pricing, refining calculations, pipeline design, and reservoir engineering. It is always measured at 60 degrees Fahrenheit (15.56 degrees Celsius) to ensure consistency.
Conversion Formula
Where API is the API gravity in degrees, and SG is the specific gravity of the petroleum liquid relative to water, both measured at 60 degrees Fahrenheit.
Crude Oil Classification
| Category | API Range | SG Range | Examples |
|---|---|---|---|
| Light | > 31.1 | < 0.870 | West Texas Intermediate, Brent |
| Medium | 22.3 - 31.1 | 0.870 - 0.920 | Arabian Light, Dubai |
| Heavy | 10 - 22.3 | 0.920 - 1.000 | Maya, Boscan |
| Extra Heavy | < 10 | > 1.000 | Orinoco, Athabasca |
FAQ
Why is lighter oil more valuable?
Lighter crudes contain more of the desirable light fractions (gasoline, diesel, jet fuel) that can be obtained through simple distillation. Heavier crudes require more complex and expensive processing (cracking, coking) to produce the same products, and yield more residual fuel oil and asphalt.
What is the API of water?
Water has a specific gravity of exactly 1.000 at 60F, which converts to 10.0 degrees API. Oils lighter than water (most crude oils) have API greater than 10 and float. Extra-heavy oils with API less than 10 are denser than water and would sink if not for other factors.
How is API gravity measured?
API gravity is measured using a hydrometer (a weighted glass float) in a sample held at 60F. The hydrometer sinks to different depths depending on the liquid density, and the API value is read from a calibrated scale on the stem. Modern labs also use digital density meters for higher precision.