"Crude oil" is a broad term covering hundreds of different grades. Buyers and refiners classify each one using two key properties: how dense it is and how much sulfur it contains. Together these decide how easily a barrel can be refined into valuable products — and therefore its price.
Crude is classified by API gravity (light vs heavy) and sulfur (sweet vs sour). Light sweet crude — like West Texas Intermediate (WTI) — is the most valuable because it is the easiest and cheapest to refine into gasoline and diesel.
Light vs heavy: API gravity
API gravity is the industry's measure of how dense an oil is relative to water, measured at 60 °F. It is calculated from specific gravity (SG):
API gravity = (141.5 / SG) − 131.5
A higher API number means a lighter, less dense oil. Lighter crudes yield more of the high-value light products (gasoline, diesel) with less processing, so they generally command higher prices.
| Grade | API gravity |
|---|---|
| Light | Greater than 31.1° |
| Medium | 22.3° – 31.1° |
| Heavy | 10° – 22.3° |
| Extra-heavy | Below 10° |
A measure of how light or heavy a crude oil is compared with water, on a scale set by the American Petroleum Institute. Higher API = lighter oil. Water is 10° API.
Sweet vs sour: sulfur content
The second axis is sulfur. Sweet crude has low sulfur and is easier and cheaper to refine because less treating is needed to meet fuel standards. Sour crude has high sulfur, which is corrosive and must be removed, so it requires more complex refining and sells at a discount.
Why grade matters
The two axes combine into four broad categories — light sweet, light sour, heavy sweet and heavy sour. Light sweet crude, exemplified by WTI and Brent, is the premium grade: easy to refine, low in sulfur, rich in valuable light products. Heavy sour crude is the cheapest, needing a complex refinery to process profitably. This is why a refinery's complexity and the crude grade it is built to handle are central to its economics.
You can convert specific gravity to API gravity instantly with our API gravity calculator.
Built by the team behind OpsFlo — field service & billing software for oilfield service companies. Capture tickets at the wellsite and bill in days, not weeks.
Frequently asked
API gravity measures how light or heavy a crude oil is compared with water, at 60 °F. It is calculated as (141.5 / specific gravity) − 131.5. A higher number means a lighter, more valuable oil.
Sweet crude has low sulfur and is cheaper to refine; sour crude has high sulfur, which is corrosive and must be removed, so it requires more complex refining and sells at a discount.
Light sweet crude — like WTI — yields more gasoline and diesel with less processing and needs little sulfur removal, so it is the easiest and cheapest to refine and commands the highest price.