API Gravity Calculator for Crude Oil
Determine the quality of crude oil by calculating its API Gravity from density.
Enter the density of the crude oil at standard temperature (60°F or 15.6°C).
Select the unit for the entered density value.
API Gravity Scale
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Understanding the API Gravity Calculator
What is API Gravity?
API gravity is an inverse measure of a petroleum liquid’s density relative to that of water. It was created by the American Petroleum Institute (API) to provide a standardized way of comparing the densities of crude oils. If a crude oil’s API gravity is greater than 10, it is lighter than water and will float; if it’s less than 10, it is heavier and will sink. This calculator helps you perform the conversion and calculate API gravity of crude oil using density inputs.
A higher API gravity indicates a lighter, less dense crude oil, which is typically more valuable. Lighter crudes are easier to refine into high-demand products like gasoline and diesel. Conversely, a lower API gravity signifies a heavier, denser crude that is more complex and expensive to process.
The API Gravity Formula
The calculation is a two-step process. First, you determine the Specific Gravity (SG) of the oil, which is its density divided by the density of water at a standard temperature (60°F or 15.6°C). Once you have the SG, you can use the standard formula. For more information on fluid properties, you might be interested in a fluid dynamics calculator.
Step 1: Calculate Specific Gravity (SG)
Step 2: Calculate API Gravity
Variables Explained
| Variable | Meaning | Unit | Typical Range (for Crude Oil) |
|---|---|---|---|
| Density of Oil | The mass per unit volume of the crude oil. | kg/m³, g/cm³, lb/gal, etc. | 700 – 1000+ kg/m³ |
| Specific Gravity (SG) | The ratio of the oil’s density to water’s density. | Dimensionless | 0.7 – 1.0+ |
| API Gravity | The standardized industry measure of oil density. | Degrees (°API) | 10° to 70° |
Practical Examples
Example 1: Medium Crude
Let’s say you have a sample of crude oil with a density of 870 kg/m³. The density of water is approximately 999 kg/m³ at 60°F.
- Inputs: Oil Density = 870 kg/m³
- SG Calculation: 870 / 999 = 0.8709
- API Gravity Calculation: (141.5 / 0.8709) – 131.5 = 162.47 – 131.5 = 30.97° API
- Result: This is a medium crude oil.
Example 2: Light Crude
Now consider a lighter crude with a density of 816 kg/m³. Exploring the crude oil classification guide can provide more context.
- Inputs: Oil Density = 816 kg/m³
- SG Calculation: 816 / 999 = 0.8168
- API Gravity Calculation: (141.5 / 0.8168) – 131.5 = 173.23 – 131.5 = 41.73° API
- Result: This is a high-quality light crude oil.
How to Use This API Gravity Calculator
- Enter Oil Density: Input the measured density of your crude oil sample into the first field.
- Select the Unit: Use the dropdown menu to choose the correct unit for the density you entered (e.g., kg/m³, lb/gal). The calculator automatically handles the conversion.
- Review the Results: The calculator will instantly display the primary API Gravity value.
- Analyze Intermediate Values: Look at the intermediate results for the Specific Gravity (SG), the oil’s classification (Light, Medium, Heavy), and its density converted to kg/m³ for easy comparison.
Key Factors That Affect API Gravity
The API gravity is fundamentally tied to the crude oil’s chemical composition. Several factors influence this:
- Temperature: Density is temperature-dependent. As crude oil is heated, it expands, its density decreases, and its API gravity increases. Standard measurements are typically corrected to 60°F (15.6°C).
- Hydrocarbon Composition: Crude oils with a higher proportion of light, short-chain paraffinic hydrocarbons will be less dense and have a higher API gravity.
- Aromatic Hydrocarbon Content: Crudes rich in heavy, complex aromatic compounds are denser and thus have a lower API gravity.
- Sulphur Content: Often, heavier crudes (lower API gravity) have a higher sulphur content (“sour crudes”), while lighter crudes (higher API gravity) have less sulphur (“sweet crudes”).
- Impurities and Sediments: Water, salts, and other non-hydrocarbon impurities increase the overall density of the fluid, leading to a lower API gravity.
- Pressure: While liquids are largely incompressible, very high pressures can slightly increase density, thereby slightly decreasing API gravity. The effect is usually minor under normal conditions.
To explore related conversions, check out our specific gravity calculator.
Frequently Asked Questions (FAQ)
- Is a higher API gravity better?
- Generally, yes. Crude oil with an API gravity between 40 and 45° commands the highest prices because it yields more high-value products like gasoline. However, if it’s too high (above 45°), it may contain shorter hydrocarbon chains that are less valuable to some refineries.
- What is the API gravity of water?
- Water has a specific gravity of 1. Plugging this into the formula gives: (141.5 / 1) – 131.5 = 10°. Therefore, water has an API gravity of 10°.
- What is the difference between specific gravity and API gravity?
- Specific gravity is a direct ratio of a substance’s density to a reference (water). API gravity is an inverse, scaled measure derived from specific gravity, created specifically for the petroleum industry to provide a more intuitive scale where “lighter” means a higher number.
- Why are 141.5 and 131.5 used in the formula?
- These constants were established by the American Petroleum Institute to create the API scale. They were derived from the older Baumé scale and adjusted to create a convenient standard for the industry.
- How do I handle different units for density?
- Our calculator does this automatically. Manually, you must first convert the oil’s density and water’s density to the same units before calculating specific gravity. For instance, if you have oil density in lb/gal, you’d divide it by the density of water in lb/gal (approx. 8.337 lb/gal).
- What is considered heavy vs. light crude?
- Classifications can vary slightly, but a common standard is: Light crude (>31.1° API), Medium crude (22.3° to 31.1° API), and Heavy crude (<22.3° API). Anything below 10° is often called extra heavy oil or bitumen.
- Can I calculate API gravity from dynamic viscosity?
- No, not directly. API gravity is a function of density, while viscosity measures a fluid’s resistance to flow. While heavy oils (low API gravity) are typically more viscous, there is no direct mathematical formula to convert between the two without knowing the density. You might find our viscosity converter useful.
- What does it mean if API gravity is less than 10?
- An API gravity of less than 10 means the substance is denser than water and will sink. This category includes extra heavy oils and bitumen.