API Gravity Explained
API gravity measures how heavy or light a petroleum liquid is compared with water. It is calculated as 141.5 divided by specific gravity at 60°F, minus 131.5. Water is 10° API. Crude above about 31° API is called light, 22–31° medium and below about 22° heavy. Lighter crude yields more gasoline and diesel, so it usually sells for more.
The API gravity formula
The American Petroleum Institute introduced the scale in 1921. It inverts specific gravity (SG), the density of the oil divided by the density of water, both at 60°F (15.6°C):
API gravity = (141.5 ÷ SG) − 131.5
To go the other way: SG = 141.5 ÷ (API gravity + 131.5)
Because the scale is inverted, a higher API number means a lighter oil. Water, with an SG of 1.0, comes out at exactly 10°. Oils above 10° float on water, and oils below 10° sink.
Worked examples
- A crude with SG 0.850: 141.5 ÷ 0.850 = 166.47, minus 131.5 = 35.0° API (light).
- A crude with SG 0.871: 141.5 ÷ 0.871 = 162.46, minus 131.5 = 31.0° API, about the gravity of Dubai crude.
- A crude with SG 0.920: 141.5 ÷ 0.920 = 153.80, minus 131.5 = 22.3° API, the boundary between medium and heavy.
Laboratories measure density with a hydrometer or digital density meter and correct it to 60°F using standard tables (ASTM D1250), because oil expands as it warms.
Light, medium and heavy thresholds
| Class | API gravity | Density (kg/m³) | Examples |
|---|---|---|---|
| Light | above 31.1° | below 870 | WTI, Brent, LLS, Murban |
| Medium | 22.3° to 31.1° | 870 to 920 | Dubai, Mars, Urals |
| Heavy | 10° to 22.3° | 920 to 1,000 | Western Canadian Select |
| Extra heavy | below 10° | above 1,000 | Undiluted bitumen, some Venezuelan crude |
These thresholds are widely used but not universal. Agencies such as the USGS and individual exchanges use slightly different ranges. Crude above about 45–50° API is often classed as condensate.
API gravity by crude grade
| Grade | API gravity | Sulfur | Notes |
|---|---|---|---|
| WTI | 37–42° | <0.42% | NYMEX delivery specification |
| Murban | ~40° | ~0.78% | ADNOC published quality |
| LLS | 34–41° (≈38.5° in practice) | ≤0.4% | Gulf Coast light sweet |
| Brent | ~38° | ~0.4% | North Sea blend |
| Dubai | ~31° | ~2% | Middle East medium sour marker |
| Mars | ~30° | ~1.8–2% | US Gulf medium sour |
| Urals | ~30° | ~1.4–1.7% | Russian export blend |
| WCS | ~20.5–21.5° | ~3–3.7% | Diluted bitumen blend |
Sources: CME, ADNOC, RBN Energy, Reuters (2022 Urals loadings), Oil Sands Magazine and Crude Monitor. Values are typical; individual cargoes vary.
Why lighter crude usually sells for more
When crude is distilled, lighter oil yields more of the valuable "light ends": naphtha and gasoline, jet fuel and diesel. Heavy crude leaves more residue, which sells for less or needs costly cokers and hydrocrackers to upgrade. A simple refinery running heavy crude would produce a large share of low-value fuel oil, so it would pay much less for it.
Heavy crudes are also usually high in sulfur, so they carry a double penalty. The sweet vs sour crude guide covers the sulfur side. Together, density and sulfur explain most of the normal price gap between, for example, Brent and Western Canadian Select.
The relationship has limits:
- Very light crude and condensate can sell at a discount because they yield mostly naphtha and too little diesel for many refineries.
- Location can outweigh quality. Inland WTI often trades below Brent despite being lighter, because of transport costs. See Brent vs WTI.
- Supply shocks reshuffle the order. In 2026 the loss of Gulf supply made medium sour grades scarce, and the IEA reported US Gulf Mars trading at a premium in March 2026.
API gravity and the barrel-to-tonne conversion
Because API gravity measures density, it also sets how many barrels make up a metric tonne. Lighter crude has more barrels per tonne. That matters wherever oil is quoted by weight, as is common in Asian and Russian trade. The figures below follow from the formula and the 158.987-litre barrel:
| API gravity | Specific gravity | Barrels per metric tonne |
|---|---|---|
| 10° | 1.000 | 6.29 |
| 20° | 0.934 | 6.73 |
| 25° | 0.904 | 6.96 |
| 30° | 0.876 | 7.18 |
| 35° | 0.850 | 7.40 |
| 40° | 0.825 | 7.62 |
| 45° | 0.802 | 7.85 |
Calculated as 1,000 kg ÷ (SG × 158.987 litres), treating water at 60°F as 1 kg per litre; results are approximate. Convert a barrel price to a tonne price on crude oil price per tonne, and barrels to litres or gallons on oil price per barrel.
API gravity FAQ
What is API gravity?
API gravity is a scale created by the American Petroleum Institute to express how heavy or light a petroleum liquid is relative to water. It equals 141.5 divided by the specific gravity at 60°F, minus 131.5. Water is 10° API, and higher numbers mean lighter oil.
What API gravity is considered light crude?
Crude with an API gravity above about 31.1° is usually classed as light. Medium crude is roughly 22.3° to 31.1°, and heavy crude is below 22.3°. Oil below 10° is extra heavy and denser than water. Definitions vary slightly between agencies and exchanges.
What is the API gravity of WTI and Brent?
Crude delivered against NYMEX WTI futures must be between 37° and 42° API. Brent blend is about 38° API. Both are light sweet crudes. Dubai is about 31°, Mars and Urals about 30°, and Western Canadian Select about 21°.
Why does lighter crude cost more?
Lighter crude yields more gasoline, jet fuel and diesel when refined and leaves less low-value residue. Heavy crude needs expensive upgrading units to make the same products. Refiners therefore pay more for light crude, although location, sulfur and short-term supply shocks can override the quality premium.
How do you convert API gravity to density?
First calculate specific gravity: 141.5 divided by the API gravity plus 131.5. Multiply by the density of water at 60°F, about 999 kg per cubic metre, to get density. For example, 30° API gives a specific gravity of 0.876, or about 875 kg per cubic metre.
Related pages
Sweet vs Sour Crude
Sweet vs sour crude oil: sulfur thresholds, why sour crude trades at a discount, refinery complexity and how IMO 2020 changed sour crude prices.
Crude Oil Benchmarks
Guide to the world's crude oil benchmarks: Brent, WTI, Dubai/Oman, Murban, OPEC basket, Urals and WCS, how each is priced and where it is used.
Price Per Tonne
Crude oil price per metric tonne from the live barrel price, barrels-per-tonne factors by grade, and why Asian and Russian trade quotes tonnes.
Oil Price Per Barrel
Today's oil price per barrel in USD for Brent and WTI, plus calculators converting the barrel price to price per gallon and per litre.