API gravity is the number that classifies a crude oil as light or heavy, and it shows up on nearly every crude assay, sales ticket, and pipeline nomination. Devised by the American Petroleum Institute, it is an inverse density scale tuned so that most crudes fall in a convenient range. This guide explains what API gravity is, the formula behind it, why 10 degrees is the dividing line, and how it is measured and used in oil and gas.
API Gravity in one line: API gravity is a scale, expressed in degrees API, that measures how light or heavy a petroleum liquid is relative to water. It is derived from specific gravity: higher API gravity means lighter, less dense oil. Water sits at 10 degrees API; oil lighter than water reads above 10 and floats, heavier reads below 10 and sinks.
API gravity is calculated from a liquid's specific gravity (SG) measured at 60 degrees Fahrenheit: API gravity = (141.5 / SG) - 131.5. Specific gravity itself is the fluid's density divided by the density of water at 60 degrees F. Because API gravity is inversely related to density, the scale runs backwards from intuition - lighter, less dense oils earn higher API numbers.
The constants are chosen so that pure water, with a specific gravity of exactly 1.0, comes out at 10 degrees API. Anything lighter than water reads above 10 (and floats on water); anything heavier reads below 10 (and sinks). Most conventional crudes land between about 10 and 45 degrees API, which is the range the scale was designed to spread out clearly.
API gravity is the primary way crude is graded. As a common rule of thumb, oil above roughly 31.1 degrees API is light crude, between about 22.3 and 31.1 is medium, and below 22.3 is heavy; extra-heavy crude and bitumen fall below 10 degrees API and are denser than water. Lighter crudes generally yield more high-value products like gasoline with less processing, so they typically command a price premium, while heavy crudes need more upgrading.
Because API gravity changes with temperature - liquids expand and become less dense when warm - it is always referenced to a standard temperature of 60 degrees F. A field measurement taken at a different temperature must be corrected back to 60 degrees F using standard API tables so that gravities are comparable and consistent across transactions.
Traditionally API gravity is read with a glass hydrometer floated in a sample cylinder of oil, with the observed reading and temperature corrected to 60 degrees F. For continuous, automated measurement, an inline densitometer reports fluid density in real time, from which API gravity is computed directly. This live density is central to converting a metered volume from flowing conditions to standard conditions and to the net-volume math on a custody-transfer skid.
On the automation side, an inline density or API-gravity value is a live tag on the flow computer or PLC. A cloud SCADA platform such as Merobix reads that digitized value over Modbus, OPC UA, or a similar protocol and displays it with flow and temperature - so operators can confirm the stream's quality and catch an off-spec gravity that signals a process upset or the wrong product in the line.
API gravity is a number that says how light or heavy an oil is compared with water. Water is 10 degrees API. Oil that floats on water reads above 10 (light crude); oil that sinks reads below 10 (heavy crude). The higher the API gravity, the lighter and generally more valuable the crude.
Specific gravity is the direct ratio of a fluid's density to water's density. API gravity is a rescaled, inverse version of specific gravity defined by the formula (141.5 / SG) - 131.5, chosen so water equals 10 degrees and lighter oils get higher numbers, which spreads common crudes over a convenient range.
Density changes with temperature, so gravities measured at different temperatures cannot be compared directly. The petroleum industry references API gravity to a standard temperature of 60 degrees F. Field readings taken at other temperatures are corrected back to 60 degrees F using standard API tables so every transaction uses a consistent basis.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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