Api Gravity Calculator
Convert petroleum density into API gravity for common crude products or a custom density. Useful for comparing light and heavy oils.
Api Gravity Calculator
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What the API gravity calculator does
API gravity is the petroleum industry's way of describing how light or heavy an oil is, on a scale built around water. This calculator converts a density into API gravity, gives the specific gravity along the way, classifies the oil as light or heavy, and tells you whether it floats on water. It has presets for common products, from gasoline to heavy fuel oil, or you can enter a custom density.
Below is what API gravity means, the equation behind it, why its scale runs backwards from density, and a worked example.
How to use it
- Pick a crude product, which fills in its density, or choose Custom to enter your own.
- Check or enter the density in kilograms per cubic metre.
- Press Calculate for the specific gravity, the API gravity, and whether the liquid floats, or Reset to clear it.
What API gravity is
API gravity, developed by the American Petroleum Institute, is a measure of how heavy a petroleum liquid is compared with water, expressed in degrees. It is really just another way of stating density, but on a scale tuned for the oil trade, where it has become the universal shorthand for crude quality. When a barrel is described as 40 degrees, that is its API gravity.
What makes it distinctive is that it runs the opposite way to density: lighter oils, which are less dense, have higher API gravity, and heavier oils have lower. This is deliberate, because in the oil business lighter crudes are generally the more valuable, yielding more petrol and diesel with less processing, so a scale where higher numbers mean better quality is convenient. A trader or operator reads 40 degrees as light and premium, 20 degrees as heavy and harder to refine, at a glance.
The equation it uses
API gravity is calculated from the specific gravity, the oil's density relative to water. The calculator first finds the specific gravity by dividing the oil's density by that of water, then applies the API formula:
API gravity = (141.5 ÷ SG) − 131.5
Because specific gravity sits in the denominator, the relationship is inverted: as the oil gets denser and its specific gravity rises, the API gravity falls. The formula is the industry standard, set out in the API Manual of Petroleum Measurement Standards.
The inverted scale and the constants
The two numbers in the formula, 141.5 and 131.5, are not arbitrary. They were chosen so that the scale lines up with the hydrometers already in wide use when the standard was set, and so that pure water lands on exactly 10 degrees. Putting a specific gravity of 1 into the formula gives 141.5 minus 131.5, which is 10, so water is the anchor point of the whole scale.
From that anchor, lighter-than-water liquids climb above 10 degrees and heavier-than-water liquids fall below it. The inversion is the scale's whole point: it turns the density of an oil, which is a slightly abstract figure, into a single number that rises with quality and value, which is exactly what the people buying and selling oil want to read.
Light, heavy, and the float line
Because water sits at 10 degrees, that value is the float line. An oil above 10 degrees API is lighter than water and floats on it; below 10 degrees it is denser than water and sinks. Heavy fuel oils can cross that line and settle in water, while most crudes and refined products stay well above it and float.
Beyond the float line, the scale is divided into quality bands. As a rough industry guide, light crude runs above about 31 degrees, medium crude around 22 to 31 degrees, heavy crude from about 10 to 22 degrees, and extra-heavy below 10. The exact boundaries vary a little between sources, so treat the labels as approximate. For a sense of scale, the well-known benchmark crudes sit in the light-to-medium range: West Texas Intermediate around 40 degrees, Brent around 38, and a heavy crude like Maya near 22.
Units and precision
The calculator takes the density in kilograms per cubic metre and reports the specific gravity as a pure number and the API gravity in degrees. By the industry definition, the specific gravity is measured relative to water at 60 degrees Fahrenheit, and the calculator uses a water density close to that standard. Results are shown to a couple of decimal places, which suits the precision of the figures used in practice. Because the scale is inverted and not linear, API gravities should not simply be averaged when blending oils.
A worked example
Take an oil with a density of 850 kilograms per cubic metre.
Its specific gravity is about 850 ÷ 997 ≈ 0.853, and the API gravity is (141.5 ÷ 0.853) − 131.5 ≈ 34.5 degrees. That places it in the light crude range and, being well above 10 degrees, it floats on water. The same calculation turns any oil's density into the degree figure the industry uses to price and classify it.
Questions people ask
What is the formula for API gravity?
API gravity = (141.5 ÷ SG) − 131.5, where SG is the specific gravity of the oil relative to water. The specific gravity comes from the oil's density divided by water's.
Why does water have an API gravity of 10?
Because the constants in the formula were chosen so it does. Putting a specific gravity of 1 into the formula gives exactly 10 degrees, making water the anchor of the scale.
Why is higher API gravity lighter oil?
Because specific gravity is in the denominator of the formula, so the scale runs backwards from density. Lighter, less dense oils get higher API values, which conveniently rise with quality and value.
When does an oil float on water?
When its API gravity is above 10 degrees, meaning it is lighter than water. Below 10 degrees it is denser than water and sinks, as some heavy fuel oils do.
References
A quick note on where this comes from. API gravity as (141.5 ÷ SG) − 131.5, with water set at 10 degrees, is the standard defined by the American Petroleum Institute, described in the Wikipedia article on API gravity and in engineering references. The SI units follow the US National Institute of Standards and Technology.
- Wikipedia, API gravity. https://en.wikipedia.org/wiki/API_gravity
- Engineers Edge, API Gravity Equations and Calculator. https://www.engineersedge.com/fluid_flow/api_gravity__15985.htm
- American Petroleum Institute, Manual of Petroleum Measurement Standards, Chapter 11.5.1 (API gravity at 60°F).
- National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
Bibek Lal Karna is a PhD student and graduate teaching assistant at the University of Mississippi, with deep interests in theoretical and gravitational physics. He is also the founder of NRCC and is strongly engaged in scientific teaching and communication. At Eon Tools, he reviews physics tools.