API gravity is the standard measure of how heavy or light a crude oil is relative to water. Devised by the American Petroleum Institute, the scale converts a crude's specific gravity into a single number that the entire oil industry uses to classify barrels. The defining feature of the scale is that it runs inversely to density: a higher API number means a lighter, less dense crude, while a lower number means a heavier, denser one. A crude with an API gravity above 10 degrees is less dense than water and will float; below 10 it is denser than water and will sink.
API gravity matters because density is closely tied to the natural yield of a barrel. Lighter crudes are richer in the short hydrocarbon chains that become high-value transport fuels — gasoline, naphtha, jet fuel and diesel — and require less expensive processing to extract them. Heavier crudes carry a larger fraction of long, dense molecules that must be broken down in costly upgrading units before they become saleable products. Because of this, API gravity is one of the two physical properties — alongside sulphur content — that determine where a crude grade prices relative to a benchmark like Brent.
The API Gravity Formula
API gravity is derived directly from specific gravity, the ratio of a crude's density to the density of water, both measured at 60 degrees Fahrenheit. The conversion is:
API gravity = (141.5 / specific gravity) − 131.5
The formula was calibrated so that pure water, which has a specific gravity of exactly 1.0, works out to 10 degrees API. Anything that floats on water therefore sits above 10; anything that sinks falls below it. The inverse relationship is the single most important thing to remember about the scale: as a crude gets physically heavier and denser, its specific gravity rises and its API number falls. This is why traders and refiners speak of "high-gravity" crude as light and "low-gravity" crude as heavy, which can seem counterintuitive until the formula is kept in mind.
The Conventional Density Bands
Crude grades are sorted into broad categories by API gravity. The boundaries are conventions rather than laws of nature, and different agencies draw them at slightly different points, but the widely used bands are:
- Light crude — above roughly 31.1 degrees API. Yields a high proportion of light products with minimal upgrading.
- Medium crude — roughly 22.3 to 31.1 degrees API. A balanced barrel that most complex refineries handle comfortably.
- Heavy crude — below roughly 22.3 degrees API. Dense and viscous, requiring conversion units to maximise light-product yield.
- Extra-heavy crude and bitumen — below 10 degrees API. So dense it sinks in water; often must be diluted or upgraded simply to flow through a pipeline.
These categories interact with geography and infrastructure. Refineries are configured around the slate of crude they expect to run, so a region rich in heavy crude tends to host complex refineries with extensive conversion capacity, while regions running mostly light crude can operate with simpler, lower-cost plants.
Why Lighter Crude Is Worth More
The economic premium for light crude comes down to yield and processing cost. A light barrel naturally contains more of the molecules that distil straight into gasoline, naphtha, jet fuel and diesel. A simple refinery can run light crude and produce a marketable product mix without heavy investment in conversion units. The same simple refinery running heavy crude would produce a large slug of low-value residual fuel oil and leave significant value on the table.
Upgrading costs. To turn the heavy bottom of a barrel into light products, a refiner needs conversion units — cokers, hydrocrackers and similar plants — that carry large capital and operating costs. Refiners equipped with that hardware can profit from cheaper heavy crude by capturing the spread between low-value feedstock and high-value output, but the option only exists where the investment has already been made. The interplay between crude quality and refinery configuration is central to refining margins.
Benchmark Grades by API Gravity
Placing the major traded grades on the scale shows how the benchmarks themselves vary in quality:
- WTI — around 40 degrees API. A light, sweet crude that is among the lightest of the major benchmarks.
- Brent — around 38 degrees API. Light and sweet, slightly heavier than WTI, and the pricing reference for much of the world's seaborne crude. You can follow the live Brent chart for the current price.
- Dubai — around 31 degrees API. A medium, sour crude that anchors pricing for barrels flowing to Asia.
- Maya — around 22 degrees API. Mexico's flagship heavy, sour grade, prized by complex Gulf Coast refineries.
- Western Canadian Select (WCS) — around 20 degrees API. A heavy, sour blend that typically trades at a steep discount reflecting both its density and pipeline constraints.
The spread between WTI or Brent at the light end and WCS or Maya at the heavy end illustrates how API gravity feeds directly into price. Lighter grades command a premium for their easy yield; heavier grades sell at a discount that compensates buyers for the upgrading they will have to perform.
Gravity Is Only Half the Picture
API gravity describes density, but it says nothing about chemical impurities. To fully classify a crude, density is always paired with sulphur content. A crude can be light and high in sulphur, or heavy and low in sulphur, and the combination determines both which refineries can profitably process it and what price it commands. A barrel that is both light and low in sulphur is the most valuable; one that is both heavy and high in sulphur is the cheapest.
Reading the two together. The benchmark grades above span this two-dimensional map: WTI and Brent are light and low-sulphur, Dubai is medium and high-sulphur, and Maya and WCS are heavy and high-sulphur. Understanding API gravity is therefore the first step; the companion measure is covered in detail in the page on sweet versus sour crude.
API Gravity in One Sentence
API gravity is the inverse density scale — calculated as 141.5 divided by specific gravity minus 131.5 — that sorts crude into light, medium, heavy and extra-heavy bands, where higher numbers mean lighter barrels that yield more high-value fuel with less upgrading and therefore command a price premium.
Continue Reading
- Sweet vs sour crude — the sulphur companion to gravity
- What is Brent crude
- What is WTI crude
- What is Maya crude
- What is Western Canadian Select
- Refining margins explained