The Physics of API Gravity & Hydrocarbon Density
In 1921, the American Petroleum Institute (API), collaborating with the National Bureau of Standards (now NIST), standardized the hydrometer scale for measuring petroleum liquids. The scale was specifically designed so that lighter crude oils carry higher numerical values, creating an intuitive commercial metric where higher API gravity generally commands premium refinery pricing due to higher yields of valuable light distillates (gasoline, jet fuel, and diesel).
The mathematical definition establishes an inverse relationship with specific gravity (SG) at the reference temperature of 60°F (15.56°C):
Where specific gravity is referenced to pure water at 60°F (ρ_water = 62.366 lb/ft³ = 999.016 kg/m³ = 8.337 lb/gal). A liquid with a specific gravity of exactly 1.0 (water) has an API gravity of 10.0° API. Any crude oil with an API gravity greater than 10.0° is lighter than water and will float.
ASTM D1250-04 Thermal Expansion & Volume Correction Factors (VCF)
Crude oil expands when heated and contracts when cooled. In custody transfer, pipeline scheduling, and fiscal metering, volumes measured at field temperatures must be adjusted to the standard reference temperature of 60°F using the Volume Correction Factor (VCF, also known as Correction for Temperature on Liquid, CTL).
Under the ASTM D1250-04 / API MPMS Chapter 11.1 standard, the volume correction factor is formulated as:
Where ΔT = T_observed(°F) - 60°F, and thermal expansion coefficient α_60 is derived from standard crude commodity constants (K_0 = 341.0957, K_1 = 0.0) based on standard base density ρ_60.
Crude Oil Classification Spectrum
| Classification | API Gravity Range | Specific Gravity (SG) | Benchmark Examples |
|---|---|---|---|
| Light Crude | > 31.1° API | < 0.870 | Brent (38.3°), WTI (39.6°), Bonny Light (35.3°) |
| Medium Crude | 22.3° to 31.1° API | 0.870 to 0.920 | Arab Heavy (27.9°), Urals (31.0°), Mars (28.8°) |
| Heavy Crude | 10.0° to 22.3° API | 0.920 to 1.000 | Maya (21.8°), Cold Lake (13.2°), Merey (16.0°) |
| Extra Heavy / Bitumen | < 10.0° API | > 1.000 | Athabasca Bitumen (8.0°), Orinoco Extra-Heavy (8.5°) |
Worked Numerical Example: Multi-Crude Blending
Two-Stream Pipeline Blend Problem:
Frequently Asked Questions (Answer-First)
Why does API gravity use 141.5 and 131.5 in its formula?
The scale derives from the French Baumé hydrometer scale (originally using 140 and 130). In the early 1900s, US hydrometer manufacturers manufactured instruments with slight scale distortions (141.5 / 131.5). Rather than recalling thousands of field hydrometers, the American Petroleum Institute officially codified the 141.5/131.5 formula in 1921.
Can crude oil have a negative API gravity?
Yes. If a petroleum substance is denser than water at 60°F (SG > 1.076), its API gravity becomes negative. Natural bitumens, coal tars, and refinery vacuum residues frequently exhibit API gravities between -2.0° and -5.0° API.
How does temperature affect custody transfer crude volume?
For every 10°F increase in temperature above 60°F, typical light-to-medium crude oil expands by approximately 0.5% in volume. Without ASTM D1250 VCF temperature correction, buyers would overpay for expanded, lower-density crude during warm weather.
Why can't API gravities be averaged linearly?
Because API gravity is inversely proportional to specific gravity (°API ∝ 1/SG). To accurately calculate a blend, volumes must be multiplied by specific gravities to calculate mass, followed by dividing total mass by total volume to find blended SG.