Degree API: The Oil Gravity Scale Explained

What Is Degree API?

Degree API (also written as °API or API gravity) is the American Petroleum Institute's standardized scale for measuring the density of crude oil and petroleum liquids relative to water, measured at 60°F (15.6°C). The formula is: °API = (141.5 / specific gravity at 60°F) − 131.5. Because the scale is inverse to density, lighter oils carry higher API numbers: water has an API gravity of 10°, typical gasoline-rich light crude is 35°–45°, and heavy bitumen may be as low as 6°–8°. API gravity is one of the most important quality parameters in crude oil pricing, refinery planning, and commercial contracts.

Key Takeaways

°API = (141.5 / SG at 60°F) − 131.5; a higher number means a lighter, less dense crude oil.
Crude oil is classified as extra heavy (<10°), heavy (10°–22.3°), medium (22.3°–31.1°), light (31.1°–45°), and extra light or condensate (>45°) by most industry conventions.
Light crude commands a price premium over heavy crude because it yields more high-value distillates — gasoline, jet fuel, diesel — per barrel without expensive upgrading.
Benchmark crudes: WTI is ~39° API, Brent ~38°, Dubai/Oman ~31°, Arab Heavy ~27°, Venezuelan Merey ~18°, and Athabasca bitumen ~8°.
Refineries designed for light crude (hydroskimming configuration) cannot efficiently process heavy crude without adding coking or hydrocracking units, a capital investment of hundreds of millions to billions of dollars.

How API Gravity Is Measured and Calculated

API gravity is derived from specific gravity (SG), which is the ratio of the density of a petroleum liquid to the density of water, both measured at 60°F (15.6°C). An SG of 0.876 corresponds to API gravity of (141.5 / 0.876) − 131.5 = 30°. The relationship is nonlinear: a small change in SG near 1.0 produces a large change in °API, while the same SG change near 0.7 produces a smaller °API difference. This means light condensates, which are near 0.70 SG, have API gravities above 70°, while bitumen at SG ~1.04 has an API of about 6°.

In the field, API gravity is measured using a hydrometer (a glass float calibrated directly in °API) or a digital density meter. Because petroleum density changes with temperature, all measurements must be corrected to the reference temperature of 60°F using ASTM Table 5A or 6A correction factors — a crude that measures 42° API at 80°F will correct to a different value at the standard 60°F reference. Pipeline metering systems and custody transfer agreements specify temperature compensation requirements precisely, as a 1°API error on a large crude delivery can represent significant monetary value.

Converting from °API back to specific gravity: SG = 141.5 / (°API + 131.5). This conversion is used constantly in reservoir engineering, fluid property correlations, and PVT (pressure-volume-temperature) analysis, where specific gravity is the native input variable for equations of state and correlations such as Standing's correlations for bubble-point pressure.

Fast Facts: Degree API

Formula: °API = (141.5 / SG at 60°F) − 131.5
Water API gravity: 10.0° (SG = 1.000)
WTI benchmark: ~39° API
Brent benchmark: ~38° API
Arab Heavy: ~27° API
Athabasca bitumen: ~8° API (upgraded synthetic ~31–34°)
Reference temperature: 60°F (15.6°C) per ASTM D1298
Measurement tool: API hydrometer or digital density meter

Field Tip:

When sampling a crude oil stream for API gravity measurement, always purge the sample line thoroughly before collecting the sample to avoid contamination with line residue from a previous crude grade. Even a small amount of a lighter or heavier product mixing into the sample can shift the measured API by 1°–2°, which may push the cargo outside the pipeline quality specification or alter the price differential applied at the custody transfer point.

API Gravity Classification and Pricing Differentials

The petroleum industry uses API gravity classifications to characterize crude quality and set price differentials. Extra heavy crude and bitumen (below ~10° API) is typically priced at a substantial discount — often $15–$30 per barrel below WTI — reflecting the cost of upgrading, diluent blending required for pipeline transport, and lower distillate yields. Heavy crude (10°–22.3° API) such as Mexican Maya (~22° API) or Venezuelan Merey (~18° API) trades at $5–$20 discounts depending on market conditions, sulfur content, and refinery coking capacity.

Medium crude (22.3°–31.1° API) trades close to benchmark prices if sulfur content is acceptable. Light crude above 31° API generally commands a premium or trades at par with WTI and Brent, as it requires less refinery processing to yield the gasoline, jet fuel, and diesel fractions most in demand globally. Very light crude and condensate above 45° API can, counterintuitively, trade at a discount in some markets if the refinery cannot handle the high naphtha content or if condensate-specific regulations apply — the U.S. crude export regulations, for example, have at various times treated condensate differently from conventional crude oil for export licensing purposes.

Refinery Implications of API Gravity

Refinery configuration is strongly matched to the API gravity of the crude slates it is designed to process. A simple hydroskimming refinery (atmospheric distillation plus naphtha reformer) achieves high margins on light crude but cannot extract value from the large heavy residue fraction that makes up most of a heavy crude barrel. A complex refinery adds vacuum distillation, fluid catalytic cracking (FCC), and hydrocracking or coking units that can convert heavy residue into lighter, more valuable products. Building coking capacity can cost $1–$4 billion, creating a long-term commitment to processing heavy crude that makes refinery operators sensitive to the light-heavy differential in crude prices.

Sulfur content (sweet versus sour) interacts with API gravity to determine crude value. The best pricing typically goes to light-sweet crudes (high API, low sulfur), which require minimal treating and yield clean distillates. Light-sour crudes require desulfurization units. Heavy-sour crudes are the most discounted, as they combine the upgrading costs of low API gravity with the treating costs of high sulfur. The combination of API gravity and sulfur content — often summarized as a quality differential matrix — is the primary tool crude traders use to value different streams against benchmark prices.

Degree API is also referred to as:

API gravity — the most common term; used interchangeably with degree API in all technical and commercial contexts
oil gravity — informal shorthand used in field and trading contexts
crude gravity — distinguishes the measurement from product gravity (e.g., condensate gravity, naphtha gravity)
specific gravity (SG) — the parent measurement from which °API is derived; SG is still used in many equations and correlations

Frequently Asked Questions About Degree API

Why does a higher API number mean a lighter oil?

The API scale is intentionally inverse to specific gravity. The formula subtracts specific gravity from a fixed number, so as specific gravity decreases (lighter fluid), the API value increases. This convention was established by the American Petroleum Institute in the 1920s so that field workers could use a simple hydrometer float — a lighter oil floats higher, giving a higher reading — without needing to perform the mathematical conversion from SG each time.

How does API gravity affect pipeline tariffs?

Most major crude oil pipelines specify acceptable API gravity ranges in their tariff schedules. Pipelines serving light crude systems (like the Enbridge Mainline or the Dakota Access Pipeline) may have minimum gravity cutoffs around 19°–20° API and maximum cutoffs of 40°–45° API. Crude outside these ranges must be blended with diluent (for heavy crude) or condensate (for very dense streams) before acceptance. Batches are gravity-stamped at origin and destination, and the pipeline operator may apply gravity adjustments to settlement volumes if the actual gravity differs from the contracted specification.

Can API gravity change over the life of a field?

Yes — crude gravity can change as a field matures. In some reservoirs, lighter fractions are produced preferentially early in field life, causing average API to decline over time. In others, gas cap expansion or solvent injection enriches the produced fluid with lighter components, increasing API. Monitoring produced crude gravity over time is a standard part of reservoir surveillance and can provide early warning of changing fluid contacts, water coning, or injection breakthrough.

Why Degree API Matters in Oil and Gas

API gravity is one of the first numbers a petroleum engineer, trader, or refiner looks at when evaluating a crude oil. It determines where a barrel fits in the global supply chain — whether it moves to a sophisticated coker refinery in the U.S. Gulf Coast, a simple topping unit in a developing country, or requires dilution just to flow through a pipeline. For producers, API gravity directly affects the netback price received at the wellhead. For nations whose fiscal revenues depend on crude export pricing, even small shifts in average API across a producing basin can translate into hundreds of millions of dollars per year in realized revenue differences.