1. Overview & Definitions
API gravity is the petroleum industry's standard measure for expressing crude oil and product density. Developed by the American Petroleum Institute in 1921, it replaced inconsistent Baumé scales and provides an inverse density measure where lighter oils have higher API values.
API Gravity
Inverse Density Scale
Higher API = lighter oil. Scale designed so water = 10° API. Range: 10–70° for petroleum.
Specific Gravity
Density Ratio
Liquid density ÷ water density, both at 60°F. Dimensionless. Written as SG 60/60.
Why It Matters
Pricing & Yield
Light crudes yield more gasoline/diesel with less processing → $5–15/bbl premium over heavy.
Custody Transfer
Volume Correction
Temperature changes volume. Correct to 60°F using VCF tables for accurate invoicing.
The API Gravity Formula
API Gravity Definition (API MPMS 11.5.1):
°API = (141.5 / SG₆₀) − 131.5
Inverse:
SG₆₀ = 141.5 / (°API + 131.5)
Where:
• °API = API gravity in degrees
• SG₆₀ = Specific gravity at 60°F/60°F (dimensionless)
Key Reference Points:
• Water: SG = 1.000 → API = 10.0°
• Light crude (SG = 0.85): API = 35.0°
• Heavy crude (SG = 0.95): API = 17.4°
Crude Oil Classification
Industry-standard thresholds per API, USGS, and EIA:
| Classification | API Gravity | SG @ 60°F | Density (kg/m³) | Examples |
|---|---|---|---|---|
| Condensate | ≥45° | ≤0.80 | ≤801 | NGL, lease condensate |
| Light | >31.1° | <0.87 | <870 | WTI (39.6°), Brent (38.3°), Arab Light (33.4°) |
| Medium | 22.3–31.1° | 0.87–0.92 | 870–920 | Mars (28.9°), Dubai (31°) |
| Heavy | 10–22.3° | 0.92–1.00 | 920–1000 | Maya (22°), WCS (20.5°) |
| Extra Heavy | <10° | >1.00 | >1000 | Orinoco (~9°), Bitumen (~8°) |
Historical note: The constants 141.5 and 131.5 were chosen in 1921 to match hydrometers already in use (which had a modulus of 141.5 instead of the Baumé scale's 140) and to place water at exactly 10° API.
2. Conversion Formulas
API ↔ Specific Gravity
API to Specific Gravity:
SG₆₀ = 141.5 / (°API + 131.5)
Examples:
35° API → SG = 141.5 / 166.5 = 0.8498
22° API → SG = 141.5 / 153.5 = 0.9218
Specific Gravity to API:
°API = (141.5 / SG₆₀) − 131.5
Examples:
SG = 0.85 → API = 166.47 − 131.5 = 35.0°
SG = 0.92 → API = 153.80 − 131.5 = 22.3°
API/SG to Density
Density from Specific Gravity:
ρ (kg/m³) = SG₆₀ × 999.016 [ASTM D1250-08 water density]
ρ (lb/gal) = SG₆₀ × 8.3372
ρ (lb/ft³) = SG₆₀ × 62.37
Direct from API Gravity:
ρ (kg/m³) = 141,361 / (°API + 131.5)
ρ (lb/gal) = 1,179.71 / (°API + 131.5)
Quick Reference Table
| °API | SG | kg/m³ | lb/gal | lb/ft³ | bbl/MT |
|---|---|---|---|---|---|
| 10 | 1.0000 | 999 | 8.34 | 62.4 | 6.29 |
| 20 | 0.9340 | 933 | 7.78 | 58.3 | 6.74 |
| 25 | 0.9042 | 903 | 7.54 | 56.4 | 6.96 |
| 30 | 0.8762 | 875 | 7.30 | 54.6 | 7.18 |
| 35 | 0.8498 | 849 | 7.08 | 53.0 | 7.41 |
| 40 | 0.8251 | 824 | 6.88 | 51.5 | 7.63 |
| 45 | 0.8017 | 801 | 6.68 | 50.0 | 7.85 |
| 50 | 0.7796 | 779 | 6.50 | 48.6 | 8.07 |
bbl/MT = barrels per metric ton. Formula: (°API + 131.5) / (141.5 × 0.159)
Blending Calculations
⚠️ Critical: API gravity does NOT blend linearly!
Correct method — blend by specific gravity, then convert:
1. Convert each crude to SG:
SG₁ = 141.5 / (API₁ + 131.5)
SG₂ = 141.5 / (API₂ + 131.5)
2. Volume-weighted average SG:
SG_blend = (V₁ × SG₁ + V₂ × SG₂) / (V₁ + V₂)
3. Convert back to API:
API_blend = (141.5 / SG_blend) − 131.5
Example: Blend 5,000 bbl of 28° API with 3,000 bbl of 38° API
SG₁ = 141.5 / 159.5 = 0.8872
SG₂ = 141.5 / 169.5 = 0.8348
SG_blend = (5000×0.8872 + 3000×0.8348) / 8000 = 0.8675
API_blend = 141.5 / 0.8675 − 131.5 = 31.6° API
(Linear averaging would incorrectly give 31.75°)
3. Volume Correction Factors (VCF)
Petroleum liquids expand when heated and contract when cooled. For fair custody transfer, measured volumes must be corrected to standard temperature (60°F) using Volume Correction Factors from API MPMS Chapter 11.1.
VCF Formula (API MPMS 11.1)
Volume Correction:
V₆₀ = V_obs × VCF
Where:
• V₆₀ = Standard volume at 60°F
• V_obs = Observed volume at measurement temperature
• VCF = Volume Correction Factor (also called CTL)
VCF Calculation:
VCF = exp[−α × ΔT × (1 + 0.8 × α × ΔT)]
Where:
• α = Thermal expansion coefficient = K₀ / ρ₆₀²
• K₀ = 613.9723 (crude oil) or 594.5418 (refined products)
• ρ₆₀ = Density at 60°F in kg/m³
• ΔT = T_obs − 60°F
Quick Rules:
• VCF < 1.0 when T > 60°F (liquid expanded → correct down)
• VCF > 1.0 when T < 60°F (liquid contracted → correct up)
• VCF = 1.0 when T = 60°F (no correction)
VCF Reference Table (Crude Oil)
| °API | 40°F | 50°F | 70°F | 80°F | 100°F |
|---|---|---|---|---|---|
| 20 (heavy) | 1.0108 | 1.0054 | 0.9946 | 0.9892 | 0.9784 |
| 30 (medium) | 1.0130 | 1.0065 | 0.9935 | 0.9870 | 0.9740 |
| 40 (light) | 1.0154 | 1.0077 | 0.9923 | 0.9846 | 0.9692 |
| 50 (condensate) | 1.0178 | 1.0089 | 0.9911 | 0.9822 | 0.9644 |
Lighter crudes have larger VCF corrections (higher thermal expansion).
Thermal Expansion by API
| °API | α (×10⁻⁴ /°F) | % Volume Change per 10°F |
|---|---|---|
| 10 | 4.0 | 0.40% |
| 20 | 4.6 | 0.46% |
| 30 | 5.3 | 0.53% |
| 40 | 6.2 | 0.62% |
| 50 | 7.2 | 0.72% |
Financial impact: For a 100,000 bbl cargo at 80°F of 35° API crude (VCF ≈ 0.9854), the standard volume is 98,540 bbl — a difference of 1,460 bbl worth ~$110,000 at $75/bbl. Accurate temperature measurement (±0.2°F) and proper VCF application are essential.
Other Correction Factors
| Factor | Abbreviation | Purpose | Typical Magnitude |
|---|---|---|---|
| Temperature on Liquid | CTL | Same as VCF — thermal expansion | 0.96–1.04 |
| Pressure on Liquid | CPL | Compressibility under pressure | 1.000–1.002 (usually negligible) |
| Combined | CTPL | CTL × CPL | Use for high-pressure metering |
| Temperature on Steel | CSW | Tank shell expansion | ~0.01%/°F |
4. Measurement Standards (ASTM)
ASTM D1298 — Hydrometer Method
Traditional laboratory method for API gravity measurement:
Procedure Summary:
1. Sample prep: Bring to ~60°F, remove water/solids, degas, pour into cylinder
2. Stabilize: Allow 30 min for temperature equilibrium; record T to ±0.1°F
3. Read: Lower hydrometer gently, let float freely, read at liquid surface (not meniscus top)
4. Correct: If T ≠ 60°F, apply VCF from API MPMS 11.1 tables
Precision:
• Repeatability: ±0.3° API
• Reproducibility: ±0.5° API
ASTM D4052 — Digital Density Meter
Modern oscillating U-tube method — preferred for lab and custody transfer QC:
- Principle: Sample fills U-tube; resonant frequency depends on mass (density)
- Sample size: 1–2 mL only
- Speed: 2–5 minutes per test
- Precision: ±0.0001 g/mL (±0.1° API)
- Temperature: Peltier-controlled to ±0.01°C
- Calibration: Two-point using air and distilled water
Method Comparison
| Method | Accuracy | Speed | Best For |
|---|---|---|---|
| Hydrometer (D1298) | ±0.5° API | 30–45 min | Field, low cost |
| Digital meter (D4052) | ±0.1° API | 2–5 min | Lab, custody transfer |
| Coriolis (inline) | ±0.2° API | Continuous | Pipeline metering |
Common Measurement Errors
| Error Source | Impact | Prevention |
|---|---|---|
| Temperature error ±1°F | ±0.05–0.08° API | Use calibrated thermometer |
| Reading meniscus top | +0.3° API | Read at liquid surface |
| Hydrometer touching wall | Biased reading | Center hydrometer, let float free |
| Entrained gas/water | Incorrect density | Degas sample, drain water |
| Wrong VCF table | 0.1–0.3° API | Use Table 5A for crude, 6A for products |
Custody transfer requirements: Both buyer and seller representatives witness sampling (per API MPMS Ch. 8). Retain split samples for referee testing. Document: observed API, temperature, VCF applied, and final API at 60°F. Measurement uncertainty directly affects revenue.
5. Practical Applications
Crude Oil Pricing
API gravity directly affects crude value through refinery yield differences:
Typical Price Differentials:
Light crude (40° API) commands premium because:
• Higher gasoline/diesel yield (45–50% vs 25–30% for heavy)
• Lower processing costs (less cracking/coking needed)
Approximate differentials vs. WTI benchmark:
• Light sweet (>35° API, <0.5% S): Benchmark price
• Medium sour (25–35° API): −$3 to −8/bbl
• Heavy sour (<25° API): −$8 to −15/bbl
• Extra heavy (<15° API): −$15 to −25/bbl
Note: Actual spreads vary with refinery config, product demand, and logistics.
Refinery Yield by API Gravity
| Crude Type | °API | Gasoline | Diesel/Jet | Heavy Fuel |
|---|---|---|---|---|
| Heavy | 20° | 25–30% | 20–25% | 30–40% |
| Medium | 30° | 35–40% | 25–30% | 15–20% |
| Light | 40° | 45–50% | 30–35% | 5–10% |
Example: Custody Transfer Calculation
Problem: Calculate payment for crude oil cargo
Given:
• Observed volume: 500,000 bbl at 82°F
• Crude: 33° API at 60°F
• Price: $72.50/bbl at 60°F
Solution:
Step 1 — Calculate VCF:
SG₆₀ = 141.5 / 164.5 = 0.8601
ρ₆₀ = 0.8601 × 999 = 859 kg/m³
α = 613.9723 / 859² = 8.32 × 10⁻⁴ /°F
ΔT = 82 − 60 = 22°F
VCF = exp[−8.32×10⁻⁴ × 22 × 1.0146] = 0.9816
Step 2 — Standard volume:
V₆₀ = 500,000 × 0.9816 = 490,800 bbl
Step 3 — Payment:
Payment = 490,800 × $72.50 = $35,583,000
Impact: Temperature correction = 9,200 bbl = $667,000 difference!
Example: Blending Two Crudes
Problem: Final API after mixing tank inventories
Given:
• Tank 1: 5,000 bbl of 28° API
• Tank 2: 3,000 bbl of 38° API
Solution:
Step 1 — Convert to SG:
SG₁ = 141.5 / 159.5 = 0.8872
SG₂ = 141.5 / 169.5 = 0.8348
Step 2 — Blend SG:
SG_blend = (5000 × 0.8872 + 3000 × 0.8348) / 8000 = 0.8675
Step 3 — Convert to API:
API_blend = 141.5 / 0.8675 − 131.5 = 31.6° API
✓ Result is between 28° and 38°, weighted toward the larger volume.
Summary: API gravity is fundamental to crude valuation, transportation, and refining. Always: (1) measure temperature accurately (±0.2°F), (2) apply correct VCF for crude vs. products, (3) blend using SG not API, and (4) document all custody transfer measurements per API MPMS standards.
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