API Gravity to Specific Gravity Converter

API gravity is the oil industry’s standard scale for describing how light or heavy a crude oil or petroleum liquid is, relative to water. It was developed by the American Petroleum Institute and is used worldwide to price crude, classify reservoirs, and estimate how a fluid will behave in surface facilities and pipelines. This converter switches instantly between API gravity and specific gravity, and reports the equivalent density in three common unit systems.

The API gravity formula

API gravity is defined directly from specific gravity measured at 60 degrees Fahrenheit relative to water at the same temperature:

API = (141.5 / SG) minus 131.5

The relationship reverses just as easily to solve for specific gravity when API is known:

SG = 141.5 / (API + 131.5)

Both constants come from the reference point of fresh water, which has a specific gravity of 1.000 and an API gravity of exactly 10.0. Anything with API above 10 floats on water and is lighter than water, and anything below 10 sinks and is heavier than water.

Density conversions

Once specific gravity is known, the calculator reports density in three practical units used across drilling, production, and refining:

• Pounds per gallon: density (ppg) = SG times 8.3372
• Kilograms per cubic meter: density (kg/m3) = SG times 999.0
• Pounds per cubic foot: density (lb/ft3) = SG times 62.366

Crude oil classification

API gravity is also used to bucket crude oils into broad market and processing categories. The boundaries used by this calculator follow common industry convention:

• Light crude: API of 31.1 or higher
• Medium crude: API from 22.3 up to 31.1
• Heavy crude: API from 10 up to 22.3
• Extra heavy crude or bitumen: API below 10

Lighter crudes generally command a price premium because they yield more high value refined products like gasoline and diesel with less processing, while heavier crudes require more complex refining and often trade at a discount.

Worked example

Consider a crude oil sample with an API gravity of 35.0, a typical light crude value.

1. SG = 141.5 / (35.0 + 131.5) = 141.5 / 166.5 = 0.8498
2. Density in ppg = 0.8498 times 8.3372 = 7.085 lb/gal
3. Density in kg/m3 = 0.8498 times 999.0 = 849.0 kg/m3
4. Since 35.0 is above 31.1, the oil classifies as light crude

These are the exact numbers the calculator returns when you click Load Example, so you can confirm the tool matches a hand calculation before trusting it with your own field data.

Why this matters

API gravity shows up constantly in drilling and production work: base oil in oil based mud systems is often specified by API gravity, crude oil sales contracts price barrels off API gravity bands, and reservoir engineers use it as a quick indicator of fluid quality and expected viscosity. Because so many downstream calculations (density, gradient, blending) rely on specific gravity rather than API gravity directly, having a fast, reliable converter between the two saves repeated unit errors.

One thing worth remembering: API gravity numbers get thrown around a lot in daily oilfield conversation, but they are always measured or corrected to 60°F. If someone hands you a hydrometer reading taken on a hot tank battery afternoon, do not plug it straight into a division order or a crude purchase contract without applying a temperature correction first. It is a small step that gets skipped more often than it should, and it is usually the reason two people arguing about a crude’s true grade are both technically right about different things.

Related Calculator and Reference

For converting drilling fluid density between ppg, specific gravity, and pressure gradient, see the Mud Weight and Hydrostatic Pressure Gradient Calculator. For background on the API gravity scale and its history, see the American Petroleum Institute.