API Gravity - Petroleum Density Classification
API gravity is an inverse scale measuring how heavy or light petroleum liquids are compared to water. This calculator converts between API gravity, specific gravity, and density, with temperature and volume corrections per ASTM D1250.
Why This Physics Calculation Matters
Why: API gravity is the petroleum industry standard for classifying crude oils. Higher API means lighter oil; it determines refining yields, transport requirements, and market pricing.
How: API gravity is calculated from specific gravity: °API = 141.5/SG - 131.5. Water (SG=1) has 10°API. Temperature corrections follow ASTM D1250 tables.
- ●Higher API = lighter liquid (inverse scale)
- ●Light crude: >31.1°API, Heavy: <22.3°API
- ●Standard reference: water at 60°F
- ●VCF corrects volume to standard temperature
🛢️ Crude Oil Samples — Click to Load
Calculation Mode
Basic Measurements
Temperature
Volume Correction
Sample Information
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
WTI crude (~40°API) is lighter than Brent (~38°API)
— EIA
Water at 60°F has SG=1.0 and 10°API by definition
— ASTM D1250
Density decreases ~0.5% per 10°F above standard
— API MPMS
Heavy oils (<22.3°API) require diluent for pipeline transport
— SPE
What is API Gravity?
API Gravity is a measure of how heavy or light a petroleum liquid is compared to water. Developed by the American Petroleum Institute (API), it's an inverse scale where higher API gravity values indicate lighter (less dense) liquids, and lower values indicate heavier (more dense) liquids. API gravity is crucial in petroleum engineering for classifying crude oils, determining refining processes, and calculating transportation requirements.
Crude Classification
API gravity determines crude oil classification: Light (35-50°API), Medium (26-35°API), Heavy (10-26°API), and Extra Heavy (<10°API).
Classifications:
- Light: 35-50°API
- Medium: 26-35°API
- Heavy: 10-26°API
- Extra Heavy: <10°API
Temperature Correction
API gravity must be corrected to standard temperature (60°F) using ASTM D1250 correction factors for accurate measurements.
Key Points:
- Standard temp: 60°F
- ASTM D1250 tables
- Critical for accuracy
Volume Correction
Petroleum volumes must be corrected to standard temperature (60°F) using Volume Correction Factors (VCF) from ASTM D1250 Table 6.
Applications:
- Inventory management
- Sales transactions
- Tax calculations
How Does API Gravity Calculation Work?
API gravity is calculated using a specific formula that relates it to specific gravity. The calculation involves measuring the density of the petroleum liquid relative to water at 60°F, then applying the API gravity formula. Temperature corrections are essential since density changes with temperature.
🔬 Calculation Process
Step-by-Step Process
- 1Measure specific gravity at measurement temperature
- 2Apply temperature correction to 60°F standard
- 3Calculate API gravity using formula: °API = (141.5/SG) - 131.5
- 4Classify crude based on API gravity range
Why This Method Works
- Standardized by API and ASTM for consistency
- Accounts for temperature effects on density
- Enables accurate volume corrections for transactions
- Provides industry-standard classification system
When to Use API Gravity Calculator
API gravity calculations are essential in petroleum engineering, refining operations, trading, and quality control. This calculator is valuable for engineers, traders, quality control technicians, and anyone working with crude oil or petroleum products.
Petroleum Trading
Essential for crude oil trading, pricing, and contract specifications. API gravity determines market value and trading premiums.
Use Cases:
- Crude oil pricing
- Contract specifications
- Quality assessment
Refining Operations
Determines optimal refining processes, product yields, and processing requirements based on crude classification.
Applications:
- Process selection
- Yield optimization
- Equipment sizing
Quality Control
Used in laboratory testing, quality assurance, and compliance with industry standards and specifications.
Standards:
- ASTM D1250
- API standards
- ISO specifications
📋 Key Takeaways
- • API gravity is an inverse scale: higher values indicate lighter (less dense) petroleum liquids
- • Standard measurement temperature is 60°F (15.556°C) per ASTM D1250
- • Crude oil classifications: Light (35-50°API), Medium (26-35°API), Heavy (10-26°API), Extra Heavy (<10°API)
- • Temperature corrections are essential for accurate measurements and volume calculations
- • WTI and Brent crude oils typically range from 38-40°API, making them premium light crudes
💡 Did You Know?
🎯 Expert Tips
💡 Always Correct to 60°F
API gravity measurements must be corrected to standard temperature (60°F) using ASTM D1250 tables for accurate classification and trading.
💡 Volume Corrections Matter
Petroleum volumes must be corrected to 60°F for accurate inventory, sales transactions, and tax calculations using Volume Correction Factors (VCF).
💡 Classification Affects Value
Light crudes (35-50°API) command premium prices due to higher yields of gasoline and diesel, while heavy crudes require specialized refining.
💡 Use Material Database
Select known crude types from the material database to automatically populate accurate attenuation coefficients and classification data.
⚖️ Crude Oil Classification Comparison
| Classification | API Gravity Range | Typical Uses | Market Value |
|---|---|---|---|
| Extra Light | 50+ °API | Gasoline, Jet Fuel | Premium |
| Light | 35-50 °API | Gasoline, Diesel, Kerosene | High |
| Medium | 26-35 °API | Diesel, Heating Oil | Standard |
| Heavy | 10-26 °API | Heavy Fuel Oil, Asphalt | Discounted |
| Extra Heavy | <10 °API | Bitumen, Upgrading Required | Low |
❓ Frequently Asked Questions
Q: What is the difference between API gravity and specific gravity?
API gravity is an inverse scale developed by the API where higher values indicate lighter liquids. Specific gravity is the ratio of a liquid's density to water's density. They are related by the formula: °API = (141.5/SG) - 131.5. Water has an API gravity of 10°API and specific gravity of 1.0.
Q: Why is 60°F the standard temperature for API gravity?
60°F (15.556°C) was established as the standard reference temperature by the API and ASTM because it represents typical ambient conditions for petroleum storage and measurement. All API gravity values must be corrected to this standard temperature for accurate comparison and classification.
Q: How does API gravity affect crude oil pricing?
Light crude oils (35-50°API) typically command premium prices because they yield more valuable products like gasoline and require less processing. Heavy crudes (10-26°API) often sell at discounts due to lower yields and the need for specialized refining equipment.
Q: What is the Volume Correction Factor (VCF)?
VCF is used to correct petroleum volumes measured at non-standard temperatures to the standard temperature of 60°F. This is critical for accurate inventory management, sales transactions, and tax calculations. VCF values are found in ASTM D1250 Table 6.
Q: Can API gravity be negative?
Yes, API gravity can be negative for very dense liquids. However, most crude oils range from about 5°API (very heavy) to 55°API (very light). Water has an API gravity of 10°API, and liquids denser than water will have lower API gravity values.
Q: How accurate are temperature corrections?
ASTM D1250 provides comprehensive correction tables that account for both API gravity and temperature differences. For most applications, corrections are accurate to within 0.1°API. However, extreme temperatures or unusual crude compositions may require specialized correction factors.
Q: What is the difference between WTI and Brent crude?
WTI (West Texas Intermediate) has an API gravity of approximately 39.6°API, while Brent crude has about 38.3°API. Both are light sweet crudes used as global pricing benchmarks. WTI is typically slightly lighter and sweeter (lower sulfur content) than Brent.
Q: Why do heavy crude oils require specialized refining?
Heavy crudes (<26°API) contain more complex hydrocarbon molecules and impurities. They require additional processing steps like coking, hydrocracking, or upgrading to break down heavy molecules into lighter, more valuable products like gasoline and diesel.
📊 Key Statistics
📚 Official Data Sources
Official API measurement standards
Last Updated: 2025-12-01
⚠️ Disclaimer
This calculator provides estimates based on standard API gravity formulas and ASTM D1250 correction factors. For commercial transactions, regulatory compliance, or critical applications, always consult official ASTM D1250 tables and verify measurements with certified laboratories. Temperature corrections are simplified approximations; actual corrections may vary based on specific crude composition and measurement conditions. This tool is for educational and estimation purposes only.
🧮 API Gravity Formulas
API Gravity from Specific Gravity
Calculate API gravity from specific gravity (SG relative to water at 60°F)
Specific Gravity from API Gravity
Calculate specific gravity from API gravity
Density from API Gravity
Calculate density using specific gravity and water density at 60°F
Temperature Correction
Correct API gravity to standard temperature (60°F) using ASTM D1250 tables
Volume Correction Factor
Correct volume to standard temperature using ASTM D1250 Table 6