Wikipedia Deep Dive

API gravity

10 min read

There's something wonderfully counterintuitive about the way oil traders talk about their product. When they say crude is "heavy," they mean it's cheap and hard to work with. When they call it "light," they mean it's valuable and flows like a dream through refineries. But here's the twist: they rarely use the word "heavy" or "light" directly. Instead, they talk about degrees of API gravity—a scale where higher numbers mean lighter oil.

Yes, you read that right. Higher API gravity means the oil is lighter. It's an inverse scale, which seems designed to confuse anyone not already in the know.

The Basic Idea

API gravity—short for American Petroleum Institute gravity—measures how heavy or light petroleum is compared to water. The magic number is 10 degrees. If oil has an API gravity greater than 10, it floats on water. Less than 10, and it sinks.

This is fundamentally about density. Oil with high API gravity is less dense than oil with low API gravity. A barrel of West Texas Intermediate crude, which has an API gravity around 39.6 degrees, is considerably less dense than a barrel of Venezuelan heavy crude at maybe 18 degrees. That difference matters enormously when you're trying to pump it through pipelines, refine it into gasoline, or simply figure out how much a tanker full of the stuff weighs.

The scale itself runs mostly between 10 and 70 degrees for petroleum products, though most crude oils cluster in the range from about 15 to 50 degrees.

A Scale Born from Error

The API gravity scale has one of those origin stories that reveals how messy real-world standards can be. In 1916, the United States National Bureau of Standards officially adopted the Baumé scale, which French chemist Antoine Baumé had developed way back in 1768. The Baumé scale was meant to measure the specific gravity of liquids lighter than water.

But there was a problem. The United States National Academy of Sciences investigated and found serious errors in how temperature and salinity were being controlled. These errors caused wild variations in published values. Making matters worse, American manufacturers had been producing hydrometers—the floating instruments used to measure density—with a mathematical modulus of 141.5 instead of Baumé's original 140.

By 1921, the incorrect hydrometers were so widespread that fixing the problem would have required replacing instruments across the entire petroleum industry. So instead, the American Petroleum Institute did what any pragmatic standards body would do: they created a new scale that matched the instruments already in use. They called it API gravity, and it used that 141.5 modulus that everyone's equipment already had.

It's a perfect example of reality dictating standards rather than the other way around.

The Mathematics

The formula for API gravity is remarkably simple. If you know the specific gravity of your oil—that is, its density relative to water—you can calculate API gravity like this:

API gravity equals 141.5 divided by the specific gravity, minus 131.5.

You can flip this around, too. If you know the API gravity and want to find specific gravity, the formula becomes:

Specific gravity equals 141.5 divided by the quantity of API gravity plus 131.5.

Let's work through an example. Suppose you have a heavy oil with a specific gravity of exactly 1.0—the same density as pure water at 60 degrees Fahrenheit. Plug that into the formula: 141.5 divided by 1.0 gives you 141.5. Subtract 131.5, and you get 10 degrees API. That's the boundary line. Anything heavier than water sinks below 10 degrees on the API scale.

Though API gravity is technically dimensionless—it's just a ratio—people in the industry refer to it as being measured in "degrees." This is because the scale is graduated on hydrometer instruments, which show degree markings.

Why This Scale Matters

Crude oil is bought and sold by the metric ton, but different crudes have different densities. A metric ton of light crude takes up more volume than a metric ton of heavy crude. So if you're trying to figure out how many barrels you're getting per ton, you need to know the API gravity.

There's a rough formula for this: barrels per metric ton approximately equals 0.1374 times the quantity of API gravity plus 7.3.

Take that West Texas Intermediate example again, with its 39.6 degree API gravity. A metric ton of it gives you about 7.6 barrels. For heavier crude with a lower API gravity, that number drops—you get fewer barrels per ton because each barrel is denser, more tightly packed with molecules.

How It's Actually Measured

There are two main ways to measure API gravity, and both start by measuring specific gravity—density relative to water.

The traditional method uses a hydrometer, a weighted glass tube that floats in liquid. You bring your petroleum sample to a standard temperature of 60 degrees Fahrenheit (15.6 degrees Celsius) to ensure consistent measurements. You select a hydrometer calibrated for the expected API range of your sample. Then you gently lower the hydrometer into the sample, let it float until it reaches equilibrium, and read the API gravity directly from the scale at the point where the liquid surface intersects the stem.

For best accuracy, you read at the bottom of the meniscus—the curved surface the liquid forms where it touches the glass. If you couldn't measure at exactly 60 degrees, you apply a correction factor to adjust your reading.

The hydrometer method is simple and cheap, which explains why it's still widely used. But it has limitations. You need a relatively large sample. It doesn't work well with highly viscous or opaque fluids. And volatile liquids require special precautions to prevent evaporation during measurement. You also need to keep your hydrometer scrupulously clean.

The modern alternative is the oscillating U-tube method. This technique is more complex but requires smaller samples and works better with difficult fluids. Both methods are detailed in standards published by ASTM International—the hydrometer method in ASTM D1298, the U-tube method in ASTM D4052.

The Price Sweet Spot

Here's where things get economically interesting. Oil with an API gravity between 40 and 45 degrees commands the highest prices. This is the sweet spot for refineries. The molecular chains in this range are long enough to be valuable but not so long that they're difficult to break down into useful products like gasoline, diesel, and jet fuel.

Above 45 degrees, the molecular chains become shorter and less valuable. Below 40 degrees, the oil is still profitable, but refineries need more sophisticated equipment to process it, which cuts into margins.

The Classification System

The petroleum industry groups crude oil into categories based on API gravity.

Light crude oil has an API gravity higher than 31.1 degrees, which translates to a density less than 870 kilograms per cubic meter. This is the easy stuff to refine—it flows readily through pipelines and yields high proportions of valuable light products.

Medium oil falls between 22.3 and 31.1 degrees API, or 870 to 920 kilograms per cubic meter. This is still quite workable for most refineries.

Heavy crude oil has an API gravity below 22.3 degrees—920 to 1,000 kilograms per cubic meter. This requires more intensive refining processes, but it's still economically viable with the right equipment.

Extra heavy oil drops below 10 degrees API, meaning it's denser than water at more than 1,000 kilograms per cubic meter. This is the challenging stuff.

Though these are the generally accepted ranges, not everyone uses exactly the same cutoffs. The United States Geological Survey, for instance, uses slightly different boundaries for its classifications.

Bitumen and the Canadian Exception

When API gravity falls below 10 degrees, petroleum enters the realm of bitumen—thick, viscous material that barely flows at all. The oil sands deposits in Alberta, Canada, produce bitumen with an API gravity around 8 degrees. At room temperature, this stuff is nearly solid.

Canadian producers have developed two strategies to make this material marketable. The first is dilution: mix the bitumen with lighter hydrocarbons to create diluted bitumen, or "dilbit," which has an API gravity below 22.3 degrees. This makes it fluid enough to pump through pipelines, though it's still in the heavy crude category.

The second strategy is upgrading: process the bitumen through additional refining steps to break down the heaviest molecules, producing synthetic crude with an API gravity of 31 to 33 degrees. This upgraded product qualifies as light crude and can command much better prices.

The Venezuelan Connection

Venezuela's oil reserves, among the largest in the world, are predominantly heavy and extra heavy crude. Much of it falls in the 15 to 20 degree API range, with some deposits even heavier. This creates both an opportunity and a challenge.

The opportunity is sheer volume—hundreds of billions of barrels. The challenge is that processing this heavy crude requires specialized refineries equipped for intensive upgrading. Not every refinery in the world can handle it profitably. The United States Gulf Coast has refineries specifically configured for Venezuelan heavy crude, which is one reason the petroleum trade relationship between the two countries has remained economically significant despite political tensions.

Heavy crude also sells at a significant discount compared to light crude. This discount reflects the additional refining costs and the lower yield of high-value light products like gasoline. When oil prices are high, the discount narrows and heavy crude becomes quite profitable. When prices fall, heavy crude producers feel the squeeze more acutely.

The Standard Temperature Problem

Here's a detail that matters more than you might think: API gravity measurements are standardized to 60 degrees Fahrenheit (15.56 degrees Celsius). But some organizations use 15 degrees Celsius (59 degrees Fahrenheit) as their standard temperature instead. That's close, but not identical.

The density of water itself changes with temperature, so when you're calculating specific gravity—which compares the oil's density to water's density—you need to use the correct density value for water at your standard temperature. According to the 2008 edition of ASTM D1250, water at 60 degrees Fahrenheit has a density of 999.016 kilograms per cubic meter. The 1980 value was 999.012 kilograms per cubic meter.

These differences look tiny, but in an industry trading millions of barrels, small errors compound into serious money.

Why Inverse?

You might wonder why the petroleum industry settled on an inverse scale where higher numbers mean lighter oil. Why not make it intuitive, where higher numbers mean heavier oil?

The answer lies in that historical accident we discussed earlier. The Baumé scale, from which API gravity descended, was designed to measure liquids lighter than water. On Baumé's original scale, higher numbers indicated lighter liquids—less dense ones that floated more readily. When the American Petroleum Institute created their new scale to match the instruments already in circulation, they preserved this inverse relationship.

By the time anyone might have considered flipping it around, the scale was so entrenched in contracts, trading systems, and industry practice that change would have been impossibly disruptive. So we're stuck with it: 40 degrees is light, 20 degrees is heavy, and newcomers to the industry just have to learn to think backwards.

It's one of those quirks that makes you realize how much of our technological world is built on historical accidents, pragmatic compromises, and the sheer inertia of established systems. The numbers themselves are arbitrary. What matters is that everyone agrees on what they mean.