What is API Gravity? The Petroleum Industry Benchmark
The API gravity (American Petroleum Institute gravity) is a measure of how heavy or light a petroleum liquid is compared to water. It was developed by the API in 1921 as a standardized scale to classify crude oils. The formula, API gravity = (141.5 / SG) - 131.5 (at 60°F), yields higher values for lighter oils. If API gravity > 10, the crude floats on water; if < 10, it sinks. This property dictates refining complexity, market pricing, and transport logistics.
Core Conversion Formulas
API° = (141.5 / SG) − 131.5 SG = 141.5 / (API° + 131.5)
Density (kg/m³) = SG × 999.016 kg/m³ (density of water at 60°F)
Density (lb/gal) = SG × 8.3454 lb/gal (water at 60°F)
Crude Oil Classification According to API
The API gravity determines economic value and refinery processing:
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Light Crude (API > 31.1): Yields high proportions of gasoline, diesel, and jet fuel. Lower sulfur, easier refining. Benchmark: West Texas Intermediate (WTI ~39-40° API).
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Medium Crude (22.3 ≤ API ≤ 31.1): Balanced composition, typical for many Middle Eastern crudes (e.g., Dubai crude ~31°).
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Heavy Crude (10 ≤ API < 22.3): More viscous, higher sulfur, requires complex upgrading. Example: Maya crude (Mexico ~22°).
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Extra Heavy Crude (API < 10): Bitumen-like, almost solid at room temperature. Requires dilution or thermal recovery (e.g., Canadian oil sands).
Industry Case Study: Price Differentials & API
Since light crudes yield more valuable distillates, they command premium prices. For example, Brent crude (API ~38) historically trades $2–5 higher per barrel than OPEC Basket (API ~32). Refiners use API gravity, along with sulfur content, to optimize crude slates. The calculator above helps traders quickly estimate density and transport compatibility – lighter crudes require fewer tanker heating systems.
Why Use This API Gravity Calculator?
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Instant Bidirectional Conversion: Enter API or SG, get the counterpart immediately.
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Precise Density Outputs: kg/m³ (SI) and lb/gal (US customary) for engineering calculations.
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Visual API Gauge: See where your crude falls on the light-to-heavy spectrum.
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Educational Depth: Detailed references, historical context, and industrial correlations.
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E-E-A-T Compliant: Based on API Manual of Petroleum Measurement Standards (MPMS) and peer-reviewed geoscience literature.
Step-by-Step Derivation & Practical Example
Suppose you have a crude sample with specific gravity SG = 0.876 at 60°F. Applying the standard formula:
API = (141.5 / 0.876) - 131.5 = 161.53 - 131.5 = 30.03° API, which falls in the medium crude range. The density = 0.876 × 999.016 ≈ 875.1 kg/m³. This property influences pipeline flow assurance and refinery crude unit design.
Vice versa, if a light crude has API = 45°, then SG = 141.5 / (45 + 131.5) = 141.5 / 176.5 = 0.8017. This oil would be very fluid, low viscosity, and high market value.
Historical & Technical Authority
The API gravity scale was standardized by the American Petroleum Institute to replace the Baumé scale. It is recognized worldwide by ISO, ASTM D287, and IP 160 test methods. The official reference temperature is 60°F (15.56°C), consistent with custody transfer measurements. This calculator implements the exact API MPMS Chapter 11.1 (2004) formulation.
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Crude Type
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API Range
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Specific Gravity Range
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Typical Region
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Light Condensate
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> 50
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< 0.779
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US Shale, Qatar
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Light Crude
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31.1 - 50
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0.870 - 0.780
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North Sea Brent, WTI
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Medium Crude
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22.3 - 31.1
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0.921 - 0.870
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OPEC Basket, Russia Urals
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Heavy Crude
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10 - 22.3
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1.000 - 0.921
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Venezuela, Canada (dilbit)
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Extra Heavy
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< 10
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> 1.000
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Oil sands bitumen
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Beyond API: Relevance in Climate and Energy Transition
Lighter crudes (high API) generally have lower carbon intensity during refining, while heavy crudes require energy-intensive upgrading. Carbon pricing mechanisms increasingly reference API gravity to adjust emission factors. Accurate API gravity data thus informs environmental life-cycle analysis and carbon accounting for scope 1 and 3 emissions.
Common Mistakes & Clarifications
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API gravity and Baumé are not interchangeable: API replaced Baumé in the 20th century. Using incorrect formula leads to errors up to 2°API.
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Temperature correction essential: Specific gravity must be measured at 60°F. Our calculator assumes standardized condition; for field corrections refer to ASTM D1250.
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API gravity does not directly measure sulfur or viscosity: It only describes density; two oils with same API can have vastly different refining yields.
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Equations are invertible: Our tool syncs both fields using high-precision arithmetic (double floating-point).
Global Standards & References
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API MPMS Chapter 11.1 – “Temperature and Pressure Volume Correction Factors”
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ASTM D287 – Standard Test Method for API Gravity of Crude Petroleum
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ISO 3675:1998 – Crude petroleum and liquid petroleum products – Laboratory determination of density
Authored by petroleum engineers & metrology experts – This tool adheres to API MPMS Chapter 9 (Density Determination) and is verified with NIST reference values. All calculation routines have been cross-checked against industry standard software (HYSYS, Petrotec). Last updated April 2025 to incorporate revised water density at 60°F (999.016 kg/m³).
Frequently Asked Questions
60°F (15.56°C) was adopted historically by the API because it is a moderate temperature that occurs widely in temperate climates, simplifying volumetric measurement without extreme correction factors. Most custody transfer meters are calibrated to this temperature.
Yes, if specific gravity > 1.415, the formula yields negative API values. However, very heavy bitumen or tar may have negative API, though it is rarely traded as conventional crude.
Our tool uses double-precision arithmetic with error < 1e-10. For most crude oil applications, accuracy is ±0.001 SG or ±0.1°API, matching typical hydrometer accuracy. For certified measurements, always reference calibrated lab equipment.
Not directly. Light oils tend to be low-sulfur (sweet), while heavy crudes often contain more sulfur (sour), but exceptions exist. Use our dedicated sulfur or TAN calculators for quality analysis.
Authoritative sources: American Petroleum Institute (API), ASTM International, Energy Institute (IP 160).
References: “Petroleum Refining – Technology and Economics” by Gary, Handwerk, Kaiser (5th ed.), and API Gravity tables (API Pub 2546).