API Gravity Calculator

Convert specific gravity to API Gravity degrees for petroleum classification.

Calculate API Gravity from specific gravity values to classify crude oil and petroleum products according to industry standards.

Examples

Click on any example to load it into the calculator.

Light Crude Oil

Light

High-quality light crude oil with low density and high API gravity.

Specific Gravity: 0.825

Medium Crude Oil

Medium

Standard medium crude oil commonly found in many oil fields.

Specific Gravity: 0.875

Heavy Crude Oil

Heavy

Heavy crude oil with high density and low API gravity.

Specific Gravity: 0.950

Water Reference

Water

Water reference point for comparison (specific gravity = 1.0).

Specific Gravity: 1.000

Other Titles
Understanding API Gravity Calculator: A Comprehensive Guide
Learn about API Gravity, its importance in the petroleum industry, and how to use this calculator to classify crude oil and petroleum products accurately.

What is API Gravity?

  • Definition
  • Historical Context
  • Industry Importance
API Gravity is a measure of how heavy or light a petroleum liquid is compared to water. It was developed by the American Petroleum Institute (API) in 1921 as a standardized way to measure the density of petroleum products. The API Gravity scale is designed so that most petroleum products have values between 10 and 70 degrees API.
The API Gravity Scale
The API Gravity scale is an inverse scale - the higher the API Gravity, the lighter the liquid. Water has an API Gravity of 10° API. Liquids lighter than water have API Gravity values greater than 10° API, while liquids heavier than water have values less than 10° API. This inverse relationship makes it intuitive for petroleum professionals to understand product characteristics.
Why API Gravity Matters
API Gravity is crucial for several reasons in the petroleum industry. It helps determine the quality and value of crude oil, influences pricing, affects transportation and storage requirements, and determines the appropriate refining processes. Higher API Gravity crude oils are generally more valuable because they yield more valuable products like gasoline and diesel fuel.
Industry Standards and Classification
The petroleum industry uses API Gravity to classify crude oils into three main categories: Light (API Gravity > 31.1°), Medium (API Gravity between 22.3° and 31.1°), and Heavy (API Gravity < 22.3°). This classification system helps traders, refiners, and transporters make informed decisions about handling and processing different types of crude oil.

API Gravity Classification Examples:

  • Light Crude Oil: 35-45° API (e.g., Brent, WTI)
  • Medium Crude Oil: 25-35° API (e.g., many Middle Eastern crudes)
  • Heavy Crude Oil: 10-25° API (e.g., Canadian oil sands, Venezuelan heavy oil)
  • Extra Heavy Crude: < 10° API (e.g., bitumen, very heavy oils)

Step-by-Step Guide to Using the Calculator

  • Input Requirements
  • Calculation Process
  • Interpreting Results
Using the API Gravity Calculator is straightforward, but understanding the input requirements and interpreting the results correctly is essential for accurate petroleum classification.
1. Understanding Specific Gravity Input
Specific Gravity is the ratio of the density of a substance to the density of water at 60°F (15.6°C). For petroleum products, this value typically ranges from about 0.5 (very light products) to 2.0 (very heavy products). Most crude oils have specific gravity values between 0.8 and 1.0. Water has a specific gravity of exactly 1.0.
2. Measurement Standards and Temperature
The specific gravity measurement must be taken at the standard temperature of 60°F (15.6°C) for accurate API Gravity calculation. Temperature variations can significantly affect density measurements, so it's crucial to use temperature-corrected values or measure at the standard temperature.
3. Calculation Process
The calculator uses the standard API Gravity formula: API Gravity = (141.5 / Specific Gravity) - 131.5. This formula converts the specific gravity measurement into the API Gravity scale. The calculation is automatic once you input the specific gravity value.
4. Interpreting the Results
The calculator provides both the numerical API Gravity value and the classification (Light, Medium, or Heavy). The API Gravity value is expressed in degrees API (°API). Higher values indicate lighter, more valuable products, while lower values indicate heavier, less valuable products.

Common Specific Gravity Values:

  • Gasoline: 0.72-0.78 (API Gravity: 58-65° API)
  • Diesel Fuel: 0.82-0.85 (API Gravity: 35-40° API)
  • Light Crude Oil: 0.80-0.85 (API Gravity: 35-45° API)
  • Heavy Crude Oil: 0.92-0.98 (API Gravity: 12-22° API)

Real-World Applications of API Gravity

  • Crude Oil Trading
  • Refining Operations
  • Transportation and Storage
API Gravity has numerous practical applications throughout the petroleum industry, from exploration and production to refining and marketing.
Crude Oil Trading and Pricing
API Gravity is a key factor in crude oil pricing. Light crude oils (high API Gravity) typically command premium prices because they yield more valuable refined products like gasoline and diesel. Heavy crude oils (low API Gravity) often trade at a discount because they require more complex and expensive refining processes. Traders use API Gravity to assess the quality and value of different crude oil grades.
Refining Process Optimization
Refineries use API Gravity to determine the optimal processing conditions for different crude oil types. Light crudes can be processed more easily and yield higher proportions of valuable light products. Heavy crudes require more sophisticated processing units like cokers and hydrocrackers to convert heavy fractions into lighter, more valuable products.
Transportation and Storage Considerations
API Gravity affects transportation and storage requirements. Heavier oils may require heating during transportation to maintain flowability. Storage tanks and pipelines must be designed to handle the specific characteristics of different API Gravity products. Safety considerations also vary with API Gravity, as lighter products are more volatile and flammable.
Quality Control and Compliance
API Gravity is used in quality control to ensure petroleum products meet specifications. Regulatory agencies often set API Gravity requirements for different product categories. Refineries and terminals use API Gravity measurements to verify product quality and ensure compliance with industry standards and regulations.

Industry Applications:

  • Crude Oil Quality Assessment and Pricing
  • Refinery Feedstock Selection and Optimization
  • Pipeline Transportation and Storage Planning
  • Regulatory Compliance and Quality Control

Common Misconceptions and Correct Methods

  • Density vs. API Gravity
  • Temperature Effects
  • Measurement Accuracy
Understanding API Gravity requires clarity on several common misconceptions and the correct methods for measurement and interpretation.
Misconception: API Gravity is the Same as Density
API Gravity is not the same as density, although they are related. API Gravity is a dimensionless scale that provides a standardized way to express the relative density of petroleum products. While density is measured in units like kg/m³ or lb/gal, API Gravity is expressed in degrees (°API) and uses an inverse scale where higher values indicate lighter products.
Temperature Effects and Standardization
A common error is neglecting temperature effects on density measurements. API Gravity calculations must use specific gravity values measured at the standard temperature of 60°F (15.6°C). Temperature variations can cause significant errors in API Gravity calculations. Always use temperature-corrected specific gravity values or ensure measurements are taken at the standard temperature.
Measurement Accuracy and Precision
Accurate API Gravity calculation requires precise specific gravity measurements. Common measurement methods include hydrometers, digital density meters, and pycnometers. Each method has specific accuracy requirements and limitations. For critical applications, use calibrated instruments and follow standard measurement procedures to ensure reliable results.
Interpreting API Gravity Values
API Gravity values should be interpreted in context. While higher API Gravity generally indicates more valuable products, other factors like sulfur content, viscosity, and chemical composition also affect product value and processing requirements. API Gravity is one of several quality indicators used in petroleum product evaluation.

Correct Measurement Practices:

  • Always measure specific gravity at 60°F (15.6°C)
  • Use calibrated instruments for accurate measurements
  • Consider temperature corrections for non-standard conditions
  • Follow industry standard measurement procedures

Mathematical Derivation and Examples

  • Formula Development
  • Calculation Examples
  • Practical Applications
The API Gravity formula was developed to provide a standardized scale for petroleum product density that would be intuitive and practical for industry use.
Formula Development and Rationale
The API Gravity formula, API = (141.5 / SG) - 131.5, was designed to create a scale where water has an API Gravity of 10° API. The constants 141.5 and 131.5 were chosen to achieve this result. The formula creates an inverse relationship where higher specific gravity (denser liquids) results in lower API Gravity values, making it intuitive for petroleum professionals.
Calculation Examples and Verification
Let's verify the formula with water: For water, SG = 1.0, so API = (141.5 / 1.0) - 131.5 = 10° API. For a light crude oil with SG = 0.85: API = (141.5 / 0.85) - 131.5 = 166.47 - 131.5 = 34.97° API. For a heavy crude oil with SG = 0.95: API = (141.5 / 0.95) - 131.5 = 148.95 - 131.5 = 17.45° API.
Reverse Calculation: API to Specific Gravity
The formula can be rearranged to calculate specific gravity from API Gravity: SG = 141.5 / (API + 131.5). This is useful when you have API Gravity values and need to determine specific gravity for other calculations or comparisons.
Practical Applications and Limitations
The API Gravity formula is widely used but has limitations. It assumes ideal behavior and may not be accurate for very light or very heavy products. For extreme values, additional corrections may be necessary. The formula is most accurate for typical petroleum products with API Gravity values between 10° and 70° API.

Calculation Examples:

  • Water (SG = 1.0): API = (141.5/1.0) - 131.5 = 10° API
  • Light Crude (SG = 0.82): API = (141.5/0.82) - 131.5 = 41.1° API
  • Heavy Crude (SG = 0.98): API = (141.5/0.98) - 131.5 = 12.9° API
  • Gasoline (SG = 0.75): API = (141.5/0.75) - 131.5 = 57.2° API