API Gravity Calculator

Understanding API Gravity in the Oil Industry

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. It is a critical metric used globally to determine the value, refining requirements, and transport characteristics of crude oil.

How API Gravity Works

Unlike normal specific gravity (SG), where water is 1.0, the API scale is inverted. If an oil's API gravity is greater than 10, it is lighter than water and will float. If it is less than 10, it is heavier than water and will sink.

Standard Classifications

The industry generally groups crude oil into four main categories based on density:

Light Crude Oil: API gravity higher than 31.1° (e.g., West Texas Intermediate). This is the most valuable as it yields the most gasoline and diesel during refining.
Medium Crude Oil: API gravity between 22.3° and 31.1°.
Heavy Crude Oil: API gravity below 22.3°. This often requires complex refining or dilution for transport.
Extra Heavy Oil / Bitumen: API gravity below 10.0°. Found in the Canadian Oil Sands and Venezuelan Orinoco Belt.

The Mathematical Formula

API gravity is derived from the Specific Gravity (SG) at 60°F (15.6°C) using the following international standard formula:

API = (141.5 / SG) - 131.5

Conversely, to find the Specific Gravity from API:

SG = 141.5 / (API + 131.5)

Why It Matters for Investors

For energy investors, the "Light-Heavy Differential" is a key profit driver. Refineries are often tuned for a specific grade of oil. When the price of heavy oil drops significantly below light oil (WTI), refineries capable of processing "sour" heavy blends can see a massive surge in crack spreads and profitability.

Formula and method

API gravity = 141.5 ÷ specific gravity at 60°F - 131.5

The petroleum-industry formula uses specific gravity relative to water with both measurements referenced to 60°F. Temperature correction may be needed when density is observed under other conditions.

Assumptions to check

Specific gravity is dimensionless and referenced to 60°F.

The sample is homogeneous and the density measurement is representative.

Classification labels are treated as conventions, not product specifications.

Worked example: crude oil with specific gravity 0.85

A petroleum sample has a specific gravity of 0.850 at the required reference temperature.

1. Divide the constant

141.5 ÷ 0.850 = 166.4706

Keep precision until the final step.

2. Subtract 131.5

166.4706 - 131.5 = 34.9706°

API gravity is reported in degrees API.

Result: The sample has an API gravity of about 35.0° API.

Frequently asked questions

What reference temperature does API gravity use?

The standard formula uses specific gravity at 60°F. Density observed at another temperature may need correction before conversion.

Does a higher API gravity mean denser oil?

No. API gravity is inversely related to specific gravity, so a higher API value indicates a less dense petroleum liquid.

Can API gravity identify crude quality by itself?

No. Sulphur, acidity, viscosity, contaminants, yield profile, and assay data also affect quality and refinery value.

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