The scale
API gravity runs inversely to density: the lighter the oil, the higher its API number. Oils with an API above 10 are lighter than water and float; below 10 they sink. The scale lets traders compare grades quickly without quoting raw density.
The formula
API gravity is derived from specific gravity (the oil's density relative to water, both measured at 60 degrees Fahrenheit) with the formula API = (141.5 / specific gravity) minus 131.5. By definition, pure water has an API of 10.
Light, medium and heavy
There is no single global standard for the boundaries, but the industry commonly uses approximate bands:
| Light crude | roughly above 31 degrees API |
|---|---|
| Medium crude | roughly 22 to 31 degrees API |
| Heavy crude | roughly below 22 degrees API |
Boundaries are approximate and vary between agencies and companies; they are a guide, not a fixed rule.
Why it matters
Lighter crudes generally yield more high-value products such as petrol, jet and diesel and command a premium, while heavier crudes need more conversion to reach the same product value. API gravity, together with sulphur, is central to how a grade is priced; see how crude pricing works and the grades we trade.
Sources & references
This article paraphrases widely established facts from the organisations below. It does not reproduce their text. Always confirm current figures against the primary source.
- American Petroleum Institute (API) — definition of API gravity
- U.S. Energy Information Administration (EIA) — crude classification