The API Gravity Calculator converts between API gravity (°API) and specific gravity for petroleum and liquid hydrocarbons. Used in oil and gas for crude oil classification, pipeline specifications, and refinery feedstock assessment — with heavy crudes below 22°API and light crudes above 31°API.
30.03
°API
0.8762
875.14
kg/m³
875.3
kg/m³
30.03
°API
0.8762
875.14
kg/m³
875.3
kg/m³
Walk into any oil trading desk or refinery planning meeting and you will hear crude described by its API gravity — "a light sweet crude at 42 API" or "a heavy sour at 18 API." API gravity is the petroleum industry's standard measure of liquid hydrocarbon density, and the calculator for API gravity converts between this industry-specific scale and specific gravity (relative density) in either direction, instantly.
API gravity is an inverse density scale developed by the American Petroleum Institute. The conversion formulas between API gravity and specific gravity (SG, measured at 60°F / 15.6°C):
°API = (141.5 / SG) − 131.5
SG = 141.5 / (°API + 131.5)
Key reference points: water has SG = 1.000 and °API = 10.0 exactly by the formula. Crude oils less dense than water (SG < 1.0) have °API above 10; crude oils denser than water have °API below 10. The formula is designed so that a 10-degree API increment corresponds to roughly equal physical property differences across the practical petroleum density range. Use this online calculator for any API gravity or specific gravity value. The octane number calculator provides related fuel quality parameters.
The petroleum industry classifies crude oil into four categories based on API gravity:
API gravity directly affects crude oil pricing. Lighter crudes command a premium because they yield more high-value light products (gasoline, jet fuel, diesel) per barrel of refinery throughput, with less processing required. The WTI-Brent price differential partly reflects API gravity differences (WTI ≈ 40°API vs. Brent ≈ 38°API) along with logistics and sulfur content factors. Heavy crude discounts can reach USD 10–30/barrel below light crude benchmarks, reflecting higher refining costs, lower product yields, and transportation challenges. The cetane number calculator and hydrocarbon calculators provide complementary petroleum property tools.
API gravity is defined at the reference temperature of 60°F (15.56°C). Crude oil expands when heated and contracts when cooled, changing the measured density and therefore the apparent API gravity. Field measurements at temperatures other than 60°F must be corrected to the reference temperature using API Table 6 (ASTM D1250) correction factors. A 1°F temperature change alters the apparent API gravity of a typical crude by approximately 0.04°API — a small correction for laboratory measurements but potentially significant for custody transfer metering where volumes are in thousands of barrels.
The relationship between API gravity and specific gravity at 60°F (15.56°C) is defined by the formula:
$$°API = \frac{141.5}{SG_{60°F}} - 131.5$$
The inverse conversion is:
$$SG_{60°F} = \frac{141.5}{°API + 131.5}$$
The density in kg/m³ can be obtained from specific gravity:
$$\rho = SG \times 999.016 \text{ kg/m}^3$$
where 999.016 kg/m³ is the density of water at 60°F (15.56°C).
The API gravity scale was chosen so that pure water has an API gravity of exactly 10° API. Petroleum liquids lighter than water (SG < 1.0) have API gravity greater than 10, while those denser than water have API gravity less than 10. The crude oil classification system defines: Light crude as API > 31.1°, Medium crude as 22.3°-31.1° API, Heavy crude as 10°-22.3° API, and Extra-heavy crude or bitumen as API < 10°.
API gravity measurements must be corrected to the standard reference temperature of 60°F using ASTM D1250 / API MPMS Chapter 11.1 volume correction factors. Accurate API gravity determination is essential for custody transfer calculations, refinery process optimization, and meeting product specifications.
Light crude oils (API > 31.1°) are the most desirable for refining because they yield higher proportions of valuable light products (gasoline, jet fuel, diesel) and require less complex processing. Benchmark crudes like West Texas Intermediate (WTI, ~39.6° API) and Brent (~38.3° API) are light crudes. Medium crudes (22.3°-31.1° API) like Arab Medium require more cracking capacity. Heavy crudes (10°-22.3° API) such as Maya crude require specialized refineries with coking and hydrocracking units. Extra-heavy crudes (< 10° API), like Canadian oil sands bitumen, are denser than water and often require diluent for pipeline transport.
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Specific gravity of 0.827 corresponds to approximately 39.6° API, characteristic of West Texas Intermediate crude oil, a premium light sweet crude.
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A specific gravity of 0.929 yields about 20.9° API, representative of Mexican Maya crude, a heavy sour crude that trades at a significant discount to light benchmarks.
API gravity is an inverse measure of petroleum liquid density relative to water, developed by the American Petroleum Institute. It is preferred in the oil industry over specific gravity because it provides a wider numerical spread for the density range of petroleum products, making it easier to distinguish between different crude types and product grades. Most petroleum liquids fall conveniently between 10 and 50° API.
The formula °API = (141.5 / SG) - 131.5 creates an inverse relationship where lower density (lower SG) produces higher API gravity. This was intentional: lighter, more valuable products have higher numbers, creating an intuitive quality scale where higher is generally better for refining economics.
Pure water at 60°F has a specific gravity of exactly 1.000 and an API gravity of exactly 10° API. Petroleum liquids with API gravity above 10 are lighter than water and will float, while those below 10 are denser and will sink in water.
Light crudes (high API) command premium prices because they produce more high-value light products (gasoline, diesel) per barrel with less complex processing. Heavy crudes sell at a discount because they require expensive upgrading equipment and yield more low-value residual products. The light-heavy differential can range from $5 to $20+ per barrel.
API gravity must be referenced to 60°F (15.56°C). If measurements are taken at other temperatures, correction factors from ASTM D1250 (API MPMS Chapter 11.1) must be applied. These correction tables account for the thermal expansion of petroleum liquids, which varies by product type and density.
Specific gravity is commonly measured using a hydrometer (ASTM D287), digital density meter (ASTM D4052), or pycnometer (ASTM D1217). Digital density meters using oscillating U-tube technology are the most accurate and widely used in modern laboratories, providing precision to 0.0001 g/mL.
Both are hydrometer scales, but they use different constants. API gravity uses 141.5 and 131.5 as constants, specifically calibrated for petroleum at 60°F. Baumé gravity exists in two versions: one for liquids lighter than water and one for heavier liquids, and is referenced to different temperatures. The petroleum industry exclusively uses the API scale.
Yes, extremely dense petroleum products like hard asphalt or vacuum residuum can have specific gravities above 1.076, resulting in negative API gravity values. For example, SG = 1.10 corresponds to -3.3° API. These materials are denser than water and represent the heaviest fraction of crude oil.
Typical ranges include: LPG (propane/butane) 100-150° API, gasoline 55-70° API, kerosene/jet fuel 40-50° API, diesel 30-40° API, lubricating oils 20-35° API, fuel oil 10-25° API, and asphalt 5-15° API. These values reflect the increasing molecular weight and density through the distillation range.
There is a general inverse correlation: lighter crudes (higher API) tend to have lower sulfur content (sweet crudes), while heavier crudes (lower API) tend to have higher sulfur (sour crudes). This is because heavier molecules concentrate sulfur compounds. However, exceptions exist: some medium crudes can be very sour, and some heavy crudes relatively sweet.
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