What is API Gravity and How Is It Measured?
The API gravity is a measure of how heavy or light a petroleum liquid is compared to water. The term API stands for American Petroleum Institute — the organization that developed this standard. API gravity is widely used to classify crude oil grades and petroleum products.
API gravity is measured using a hydrometer or an oscillating U-tube instrument conforming to ASTM D1298 or D4052 standards. The instrument measures the specific gravity of the liquid at 60 °F (15.56 °C), which is then converted to °API using the formula below.
How to Calculate API Gravity from Specific Gravity
The relationship between API gravity and specific gravity (SG) is defined by two equations:
SG = 141.5 / (API + 131.5) API = (141.5 / SG) − 131.5
where specific gravity is the ratio of the liquid's density to the density of water at 60 °F (≈ 999 kg/m³). You can also calculate SG directly from density:
SG = ρliquid / ρwater (ρwater = 999 kg/m³)
Example: Using the API Gravity Calculator
Question: What is the API gravity of gasoline with a density of 750 kg/m³?
- Enter the density as 750 kg/m³ in the Metric mode.
- The calculator computes: SG = 750 / 999 = 0.7508
- API = (141.5 / 0.7508) − 131.5 = 188.46 − 131.5 = 56.96 °API
Since 56.96 °API > 42, gasoline is classified as Ultra Light / Condensate.
API Gravity Classification
| Type | API Range (°API) | Behavior in Water | Typical Examples |
|---|---|---|---|
| Extra Heavy Oil | < 10 °API | Sinks (denser than water) | Tar sands, bitumen |
| Heavy Crude Oil | 10 – 22.3 °API | Sinks or barely floats | Venezuelan crude, residual fuel oil |
| Medium Crude Oil | 22.3 – 31.1 °API | Floats | West Texas Intermediate blends |
| Light Crude Oil | 31.1 – 42 °API | Floats easily | Brent Crude, WTI, sweet crude |
| Ultra Light / Condensate | > 42 °API | Floats (much less dense) | Gasoline, naphtha, condensate |
Key reference: Pure water at 60 °F has an API gravity of exactly 10 °API. Any liquid with API > 10 floats on water; any liquid with API < 10 sinks.
Unit Systems Supported
This calculator accepts density input in both the metric and American (US customary) systems:
- Metric: kg/m³ and g/cm³
- American: lb/gal (US gallons) and lb/ft³
- Dimensionless: Specific gravity (SG) — works with both systems
Results are always shown in all unit systems simultaneously.
Frequently Asked Questions
What does a higher API gravity mean?
A higher API gravity means the oil is lighter (less dense). Light crude (high °API) is generally more valuable because it yields more gasoline and other high-value products during refining.
What is the API gravity of water?
Water at 60 °F has a specific gravity of 1.0, which gives API = (141.5/1.0) − 131.5 = 10 °API. This is the reference point: crude oil with API > 10 floats on water.
What is the API gravity of gasoline?
Gasoline typically has a density of about 720–780 kg/m³, giving an API gravity of approximately 52–64 °API — well above the "ultra light" threshold of 42 °API.
Can API gravity be negative?
Yes. Very dense liquids (SG > 1.078) yield a negative API gravity. For example, mercury (SG ≈ 13.6) has an API gravity of about −121 °API.
Is API gravity the same as specific gravity?
No, but they are related. Specific gravity is a dimensionless ratio (liquid density / water density), while API gravity is a transformed scale that makes it easier to compare crude oil grades. A higher SG corresponds to a lower API gravity.