API Gravity
API gravity is a specific gravity scale developed by the American Petroleum Institute (API) and the National Institute of Standards and Technology (NIST) that expresses the density of crude oil and petroleum liquids relative to water, using a formula that inverts the conventional specific gravity relationship so that lighter (less dense) oils have higher numerical API gravity values. The formula is: °API = (141.5 / SG₆₀) − 131.5, where SG₆₀ is the specific gravity of the liquid at 60 degrees Fahrenheit (15.56 degrees Celsius) relative to water at the same temperature. Water has a specific gravity of exactly 1.000 and an API gravity of exactly 10.0 degrees; crude oils lighter than water have API gravities above 10 degrees and those heavier than water (which rarely occur in nature) would have API gravities below 10 degrees. The API gravity scale ranges from about 5 to 6 degrees at the heaviest end (Athabasca oil sands bitumen) to above 60 degrees for the lightest condensates and natural gas liquids. The API gravity system was adopted in the early 20th century to standardise crude oil characterisation for trading, transportation, and refinery feedstock qualification, and it remains the universal commercial metric by which crude quality is described worldwide. API gravity is a single-parameter surrogate for crude oil quality because density correlates reasonably well with the yield of valuable light products (gasoline, jet fuel, diesel) in a given refinery configuration: lighter crude (higher °API) generally contains more of the lighter distillate fractions that require less processing to convert into transport fuels, while heavier crude (lower °API) contains more resid and requires more intensive (and costly) conversion through cracking, coking, or hydroprocessing to produce the same product slate. This correlation makes API gravity a primary determinant of crude oil pricing differentials: benchmark crudes like West Texas Intermediate (WTI) at 39 to 41 °API and Brent Blend at 38 to 39 °API command premium prices over heavy Canadian crude blends like Western Canadian Select (WCS) at approximately 20.5 °API, reflecting the lower refinery yield and higher processing cost of heavy sour crude compared to light sweet benchmark crudes.
Key Takeaways
- The API gravity formula inverts specific gravity so that lighter oils carry higher numbers, a counterintuitive but commercially convenient convention: At specific gravity 0.85 (a typical light crude oil), the API gravity is 141.5/0.85 − 131.5 = 35.4 °API. At SG = 1.00 (water), °API = 10.0. At SG = 0.80 (a light condensate), °API = 45.4. The effect of the inversion is that a one-point increase in API gravity corresponds to a small decrease in specific gravity: near the WTI benchmark of 40 °API (SG ≈ 0.825), a one-degree increase in API gravity corresponds to a density decrease of approximately 1.5 kg/m³, while near the WCS heavy crude at 20 °API (SG ≈ 0.934), a one-degree increase corresponds to approximately 2.7 kg/m³ density change. Temperature must be accounted for in any API gravity measurement because petroleum liquid density changes with temperature; all reported API gravities are corrected to the reference temperature of 60 degrees Fahrenheit using petroleum liquid density correction tables (ASTM D1250, also called the Petroleum Measurement Tables or API/ASTM/IP tables), regardless of the temperature at which the actual hydrometer or densitometer measurement was made. A lab technician measuring a crude oil sample at 20 degrees Celsius and finding SG = 0.842 would correct this to 60°F using the ASTM D1250 tables to find SG₆₀ ≈ 0.849 and report API gravity of 141.5/0.849 − 131.5 = 35.7 °API as the standard reference-temperature value.
- API gravity classification divides crude oils into light, medium, heavy, and extra-heavy categories with distinct commercial and refinery implications: The industry generally classifies crude oils as: light (above 31.1 °API, SG below 0.871), medium (22.3 to 31.1 °API, SG 0.871 to 0.920), heavy (10 to 22.3 °API, SG 0.920 to 1.000), and extra-heavy or bitumen (below 10 °API, SG above 1.000). In the Western Canada Sedimentary Basin, the crude quality spectrum spans nearly this entire range. Cardium and Viking oil from central Alberta is typically 38 to 44 °API light sweet crude with low sulphur (0.2 to 0.4 percent by weight), commanding prices at or near the Edmonton par benchmark (itself linked to WTI). Peace River heavy oil from the Peace River arch averages 12 to 22 °API with sulphur content of 2 to 4 percent and must be blended or upgraded before it is pipelineable. Athabasca and Cold Lake bitumen in situ is typically 8 to 10 °API with 4 to 5 percent sulphur, requiring diluent addition (condensate or synthetic crude) to achieve the minimum 19 to 21 °API and maximum 350 to 400 cSt viscosity required for pipeline shipment as diluted bitumen (dilbit). The Western Canadian Select (WCS) benchmark crude blend, a mixture of heavy oil, bitumen blend, and conventional oil averaging approximately 20.5 °API and 3.4 percent sulphur, trades at a discount of USD 10 to 25 per barrel below WTI, with the discount fluctuating based on pipeline takeaway capacity, refinery demand, and the WTI-WCS quality differential at the time.
- API gravity strongly influences refinery crude oil feedstock economics and product yield calculations: Refineries characterise incoming crude by API gravity alongside sulphur content (sweet below 0.5 percent, sour above 0.5 percent) and key inspection properties (viscosity, pour point, Reid vapour pressure, total acid number). A typical light sweet crude at 38 °API run through an upgrading refinery equipped with fluid catalytic cracking (FCC) or hydrocracking will yield approximately 45 to 55 percent gasoline blendstocks, 20 to 25 percent diesel, and 5 to 10 percent residual fuel oil. A heavy sour crude at 22 °API through the same refinery may yield only 25 to 35 percent gasoline blendstocks, 15 to 20 percent diesel, and 30 to 40 percent residual fuel or coker feed, reflecting the lower proportion of lighter fractions and higher conversion requirement. The API gravity of the refinery's crude slate directly enters the refinery's linear programming (LP) optimisation model as a constraint on product yield and throughput capacity. Refineries with extensive conversion capacity (cokers, hydrocrackers) can process heavy crude economically and capture the quality discount as margin, while refineries designed for light crude processing lose economic value when heavy crude is substituted and their conversion units are underutilised.
- API gravity differentials between crude benchmarks drive pipeline economics and export marketing decisions for WCSB producers: The WCS-WTI price differential is the single most commercially important metric for Alberta heavy oil producers and is directly linked to API gravity difference between the two benchmarks. WCS at 20.5 °API trades at a discount to WTI at 40 °API for two reasons: quality differential (heavier crude requires more refinery investment to produce equivalent product volumes) and transportation differential (heavier crude has higher pipeline tariffs and may require diluent that adds cost). When the WCS-WTI differential widens beyond USD 15 to 20/bbl, heavy oil projects become marginally economic or uneconomic for some producers, while refiners benefit from low-cost feedstock. Narrowing differentials (USD 8 to 12/bbl) reflect either higher demand for heavy crude in US Midwest refineries or pipeline constraints reducing supply at the WCS delivery point. API gravity affects transportation costs because heavy crude requires either heated pipelines (to reduce viscosity) or diluent blending (to meet viscosity specifications), with dilbit typically requiring a diluent ratio of 25 to 30 percent condensate by volume to achieve the pipelineable viscosity specification, effectively shipping a barrel of condensate for every 2.5 to 3 barrels of bitumen, a significant cost relative to pipeline tariff for conventional light crude.
- Condensate and natural gas liquids have API gravities well above 50 degrees and serve as diluents for heavy oil transportation: Natural gas liquids (NGL) recovered from natural gas processing plants and condensate recovered from retrograde condensate gas reservoirs typically have API gravities of 50 to 80 degrees, reflecting their low molecular weight (pentane to octane-range hydrocarbons). Condensate from Montney and Duvernay liquids-rich plays in northwest Alberta and northeast BC is often produced at 60 to 70 °API and commands premium pricing above WTI as a diluent for bitumen transportation. The economic value of high-API condensate as a diluent is a major driver of Montney condensate exploration and development strategy: a 65 °API condensate with minimal sulphur and very low viscosity reduces the diluent requirement for dilbit blending and can be shipped back to oil sands upgraders in northern Alberta via pipeline as a return diluent shipment. The circular economics of condensate (produced in the Montney, shipped north as diluent for bitumen, mixed into dilbit, shipped south to refineries, condensate separated at the refinery and shipped back north as diluent again) represents one of the more complex hydrocarbon logistics chains in the WCSB.
API Gravity Measurement, Benchmarks, and Crude Oil Quality Assessment
The physical measurement of API gravity at the wellsite or lease automatic custody transfer (LACT) unit uses either a glass hydrometer (ASTM D1298) or an electronic densitometer (ASTM D4052), both calibrated to NIST-traceable reference standards at the reference temperature of 60 degrees Fahrenheit. The hydrometer method floats a weighted glass bulb in the crude sample in a graduated cylinder; the depth to which it sinks indicates the density (and therefore the API gravity at the observed temperature). Because crude oil is measured at ambient temperature rather than the reference 60°F, the observed API gravity must be corrected using ASTM D1250 petroleum measurement tables or the equivalent software routine in SCADA or allocation systems. For custody transfer purposes (the measurement that determines the quantity of crude oil delivered to a pipeline or sold to a buyer), electronic densitometers with automatic temperature compensation are the standard instrument, providing API gravity readings accurate to ± 0.1 degrees at the reference temperature.
Crude oil benchmarks are defined crude streams of known API gravity, sulphur content, and physical properties that serve as pricing reference points for oil trading. WTI (West Texas Intermediate) is the North American light sweet benchmark: 39 to 41 °API, less than 0.4 percent sulphur, delivery at Cushing, Oklahoma. Brent Blend (North Sea benchmark): 38 to 39 °API, 0.4 to 0.5 percent sulphur, a blend of crude from multiple North Sea fields. Dubai/Oman (Middle East sour benchmark): approximately 31 °API, 2.0 percent sulphur. The Edmonton Mixed Sweet blend (MSW) is the Canadian light sweet benchmark: 40 °API, less than 0.5 percent sulphur, delivery at Edmonton. WCS (Western Canadian Select) is the Canadian heavy blend benchmark: 20.5 °API, 3.5 percent sulphur, delivery at Hardisty, Alberta. Each benchmark's API gravity is part of its defining specification; a crude stream whose API gravity changes (due to changing field mix or diluent blending) may no longer qualify as the named benchmark stream and must be priced differently.
Crude oil quality certificates (COSQ or "certificates of quality") issued by pipeline operators or crude oil purchasers document the API gravity, sulphur content, BSW (basic sediment and water content), Reid vapour pressure, and viscosity of each crude stream at the point of custody transfer. These certificates are the basis for price adjustments (quality adjusters or differentials) applied to the crude oil purchase price when the actual API gravity of the delivered crude differs from the benchmark specification. A WCSB mixed sweet crude producer delivering crude at 37.5 °API to the Edmonton pipeline system may receive a quality adjustment of minus CAD 0.05 to 0.15 per barrel per degree below 40 °API (the Edmonton par specification), while a producer delivering at 41 °API may receive a small premium above par, depending on the pipeline tariff schedule in effect.
API gravity is also used to classify petroleum for tax, royalty, and regulatory purposes in Canada. Alberta's oil royalty system distinguishes between conventional oil and oil sands by API gravity: crude produced from mineable oil sands operations (Athabasca surface mines) is taxed differently from in-situ thermal production (SAGD, CSS), which is in turn taxed differently from conventional heavy oil production from naturally occurring sub-22 °API crude reservoirs. The AER's crude oil characterisation for well licence purposes also uses API gravity to classify production: wells producing oil above 31.1 °API are classified as light oil producers (qualifying for more favourable economic thresholds in some circumstances), while those producing below 22.3 °API are heavy oil producers subject to different royalty and reporting frameworks. The API gravity measured in the production test conducted at first production is recorded in the AER's production database as the reference quality specification for that well's production reporting.