Convert between API gravity and specific gravity for petroleum liquids. Enter either an API gravity value or a specific gravity value and get an instant conversion in both directions. You can also input density in kg/m³ to calculate API gravity directly. Select a preset crude product or enter a custom value to classify your petroleum liquid.
API Gravity
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Specific Gravity
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Density
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Crude Classification
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API gravity is a measure of how heavy or light a petroleum liquid is compared to water, developed by the American Petroleum Institute (API). It is expressed in degrees (° API). If the API gravity is greater than 10, the liquid is lighter than water and floats; if less than 10, it sinks.
The formula is: API Gravity = (141.5 / Specific Gravity) − 131.5. For example, a liquid with a specific gravity of 1.0 (same as water) has an API gravity of 10°. The formula was standardized by the American Petroleum Institute and is universally used in the oil industry.
Rearranging the API formula gives: Specific Gravity = 141.5 / (131.5 + API Gravity). So a crude oil with 35° API has a specific gravity of 141.5 / (131.5 + 35) = 0.8498.
Specific gravity is measured using a hydrometer — a sealed glass tube that floats at different levels depending on the density of the liquid. Modern labs also use oscillating U-tube density meters for more precise measurements. Both methods measure density relative to water at 60°F (15.6°C).
You can estimate API gravity by measuring the specific gravity or density of the petroleum liquid using a hydrometer, then applying the formula: API = (141.5 / SG) − 131.5. Our calculator automates this conversion in both directions.
Kerosene typically has an API gravity of around 40–50° API, corresponding to a specific gravity of approximately 0.78–0.82. This classifies it as a light petroleum product, lighter than diesel but heavier than gasoline.
Crude oil is generally classified as: Extra Heavy (below 10° API), Heavy (10–22.3° API), Medium (22.3–31.1° API), Light (31.1–45° API), and Extra Light or condensate (above 45° API). Light crude oils are more valuable because they yield more gasoline and other high-value products.
API gravity is a key quality indicator for crude oil. Higher API gravity generally means lighter, more valuable crude that produces more refined products like gasoline. It affects pricing, refinery processing costs, pipeline transport characteristics, and international trade classifications.