Input Parameters
Crude Oil Classification
Industry thresholds per API, USGS & EIA:
| Type | °API | ρ (kg/m³) |
|---|---|---|
| Condensate | ≥45° | <800 |
| Light | >31.1° | <870 |
| Medium | 22.3–31.1° | 870–920 |
| Heavy | 10–22.3° | 920–1000 |
| Bitumen | <10° | >1000 |
Benchmark Crudes
Reference values for major trading grades:
API gravity is inversely proportional to density — lighter crudes have higher API values. The scale was designed so water equals exactly 10° API, and typical petroleum products range from 10° (heavy oils) to 70° (light condensates).
Do not linearly average API gravity values when blending crudes. The nonlinear relationship between API and SG means you must: (1) convert each crude's API to SG, (2) calculate volume-weighted average SG, then (3) convert back to API. Linear averaging of °API introduces error, though it's sometimes accepted for small blends where accuracy isn't critical.
For cargo and trading calculations: bbl/MT = (°API + 131.5) ÷ (141.5 × 0.159)
Example: WTI (39.6° API) ≈ 7.6 bbl/MT | Heavy crude (22° API) ≈ 6.8 bbl/MT
📚 Learn the Theory
API gravity derivation, temperature corrections (CTL/VCF), and volume correction factors
API Gravity Classification Chart
Standards & References
-
API MPMS Ch. 11.1
Temperature & Pressure Volume Correction Factors (CTL, CPL, CTPL) -
API MPMS Ch. 11.5.1
Conversions of API Gravity at 60°F -
ASTM D1250-08
Petroleum Measurement Tables (VCF calculations) -
ASTM D287
API Gravity by Hydrometer -
ASTM D1298 / D4052
Density by Hydrometer & Digital Meter
Technical Notes
All values are at 60°F (15.56°C) and 1 atm (101.325 kPa). ASTM D1250-2008 specifies water density at 60°F as 999.016 kg/m³ (8.3372 lb/gal). The 1980 edition used 999.012 kg/m³.
Crude with API >10° floats on water; <10° sinks. Most petroleum products range 10–70° API. Extra-light condensates can exceed 60°.
Crudes at 40–45° API typically command highest prices due to optimal gasoline/diesel yields. Above 45°, excessive light ends (LPG) reduce value. Below 22°, upgrading costs increase significantly.
API gravity increases with temperature (as density decreases). For custody transfer at T ≠ 60°F, apply CTL factors from API MPMS 11.1 to correct observed gravity to standard conditions.
Hydrometer (ASTM D287/D1298): traditional, requires meniscus and temperature corrections. Oscillating U-tube (ASTM D4052/D5002): digital, faster, more precise — preferred for lab and inline measurement.
Quick Reference Table
| °API | SG | kg/m³ | lb/gal | bbl/MT |
|---|---|---|---|---|
| 60 | 0.7389 | 738 | 6.16 | 8.5 |
| 50 | 0.7796 | 779 | 6.50 | 8.1 |
| 40 | 0.8251 | 824 | 6.88 | 7.6 |
| 35 | 0.8498 | 849 | 7.08 | 7.4 |
| 30 | 0.8762 | 875 | 7.30 | 7.2 |
| 25 | 0.9042 | 903 | 7.54 | 7.0 |
| 20 | 0.9340 | 933 | 7.78 | 6.7 |
| 10 | 1.0000 | 999 | 8.34 | 6.3 |
Formulas
The API scale originated from the Baumé scale (France, 1768). In 1921, API standardized the modulus at 141.5 (vs. Baumé's 140) to match hydrometers already in widespread U.S. use. The 131.5 constant places water at exactly 10° API.