API gravity is an inverse measure of petroleum liquid density relative to water, developed by the American Petroleum Institute. It uses the formula °API = (141.5 / SG) - 131.5, where SG is specific gravity at 60°F. Water has an API gravity of 10°, and lighter oils have higher API values.

What is the difference between light and heavy crude oil?

Per industry standards (API/USGS/EIA): Light crude has API gravity >31.1° (density 1000 kg/m³).

Why can't you average API gravity values when blending crudes?

API gravity has a nonlinear relationship with density (it's inversely proportional to specific gravity). When blending crudes, you must average the specific gravities (weighted by volume or mass), then convert the blended SG back to API gravity. Linear averaging of API values introduces error, though it's sometimes accepted for small blends where accuracy isn't critical.

API Gravity Calculator | Specific Gravity Conversion

Input Parameters

Calculation Direction

Specific Gravity (60°F/60°F)

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Petroleum range: 0.50 – 1.10

API Gravity

°API

ASTM D1250 range: −10° to 100°

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

Note: Thresholds vary slightly by source. Values above are most widely used.

Benchmark Crudes

Reference values for major trading grades:

WTI 39.6°

Brent 38.3°

Arab Light 33.4°

Dubai 31.0°

Maya 22.0°

Orinoco ~9°

Reference: Water = 10.0° API (SG = 1.000 @ 60°F)

Key Concept

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).

Blending Note

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.

📦 Barrels per Metric Ton

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

Read Engineering Guide →

Formulas

°API = (141.5 ÷ SG) − 131.5

SG = 141.5 ÷ (°API + 131.5)

°API = API Gravity (degrees)

SG = Specific Gravity @ 60°F/60°F

📜 Historical Note

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.

Standards & References

API ASTM IP ISO

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

Reference Conditions

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.

Float/Sink Threshold

Crude with API >10° floats on water; <10° sinks. Most petroleum products range 10–70° API. Extra-light condensates can exceed 60°.

Pricing & Refinery Yield

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.

Temperature Effects

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.

Measurement Methods

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

bbl/MT = barrels per metric ton (1 bbl = 42 US gal = 0.159 m³)

API Gravity Calculator

Input Parameters

Crude Oil Classification

Benchmark Crudes

📚 Learn the Theory

Results

Formulas

Standards & References

Technical Notes

Quick Reference Table

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