How Much Does Oil Weigh? From Density to API Gravity

A standard barrel of crude oil (42 US gallons) weighs somewhere between about 275 and 350 pounds, depending on the type of crude. That wide range exists because “oil” is not one substance with a fixed density. Light crudes from West Texas weigh less per gallon than heavy crudes from Venezuela, and refined products like gasoline weigh less still. The relationship between an oil’s density and its weight is captured by a petroleum-industry scale called API gravity, which flips the intuition: the lighter the oil, the higher the API number. Understanding that scale, and the handful of factors that shift oil density in the real world, is how you move from a vague “it depends” to a useful answer.

Why Oil Weight Varies So Much

Oil is a mixture of hundreds of different hydrocarbon molecules, ranging from tiny, lightweight ones like methane and pentane to enormous, heavy ones like asphaltenes. The proportions of those molecules determine the mixture’s overall density. A crude oil rich in small, simple hydrocarbons will be lighter than one loaded with large, complex molecules. Research on North Sea crudes has shown that you can predict the density of a crude oil fraction by fraction, calculating average densities and molecular weights for each boiling-range group and then building up a picture of the whole fluid.

This molecular diversity is why different oils can feel so different. Gasoline, which is a refined fraction dominated by small hydrocarbons, has a density of roughly 0.72 to 0.78 g/mL. A typical light crude oil sits around 0.82 to 0.87 g/mL. Heavy crude can reach 0.92 to 1.0 g/mL, and some extra-heavy bitumens exceed the density of water entirely. Cooking oils fall in their own range, generally between 0.91 and 0.93 g/mL, which is why olive oil floats on water but sinks noticeably faster in alcohol.

Practical Weight References for Common Oils

If you just need a number for a project, a shipment, or a back-of-the-envelope calculation, here are the ballpark weights per US gallon at around room temperature:

Gasoline: about 6.0 to 6.3 pounds per gallon
Diesel fuel: about 6.9 to 7.1 pounds per gallon
Light crude oil (API 35–45): about 6.5 to 7.0 pounds per gallon
Medium crude oil (API 22–35): about 7.0 to 7.8 pounds per gallon
Heavy crude oil (API 10–22): about 7.8 to 8.3 pounds per gallon
Motor oil (SAE 10W-30): about 7.0 to 7.4 pounds per gallon
Olive oil: about 7.6 pounds per gallon
Water (for comparison): 8.34 pounds per gallon

All of these are lighter than water, which is why most petroleum products float when spilled. But the gap between the lightest gasoline and the heaviest crude is substantial, about two pounds per gallon, and that difference has real consequences for everything from tanker loading to pipeline flow rates.

What API Gravity Actually Tells You

The American Petroleum Institute created the API gravity scale in the early twentieth century as a standardized way to classify crude oils by density. Instead of reporting density directly in grams per milliliter, the scale converts specific gravity (density relative to water at 60 °F) into a number that runs in the opposite direction: lighter oils get higher API numbers, and heavier oils get lower ones.

The formula is straightforward in concept. You take the specific gravity of the oil at 60 °F, divide it into a constant, and subtract another constant. The result is a scale where water itself sits at exactly 10° API. Any oil above 10° API is lighter than water and will float. Any oil below 10° API is denser than water and will sink. In practice, most traded crude oils fall between about 15° API on the heavy end and 45° API on the light end, with a few ultra-light condensates reaching into the 50s and 60s.

The industry generally splits crudes into three buckets based on API gravity. Light crudes, above roughly 31° API, are the most valuable because they yield a higher proportion of gasoline, diesel, and jet fuel when refined. Medium crudes fall between about 22° and 31° API. Heavy crudes, below 22° API, require more complex and expensive refining. The benchmark West Texas Intermediate (WTI) crude typically registers around 39° to 40° API, while Brent crude sits near 38° API. On the heavy end, Venezuelan Boscan crude can be as low as 10° API, barely lighter than water.

One thing the API number does not tell you is chemical composition. Two crudes with the same API gravity can have very different sulfur content, wax content, and proportions of aromatics versus paraffins. API gravity is a density measure, not a quality score, although density and quality are loosely correlated because lighter crudes tend to contain less sulfur and more of the molecules refineries want.

How Temperature Shifts Oil Weight

Temperature is the single biggest variable that moves oil density in everyday conditions. Like most liquids, oil expands when heated and contracts when cooled. That means a gallon of crude oil measured on a hot summer day weighs slightly less than the same gallon measured in winter, because the liquid has expanded and some of the mass that was in that gallon has pushed outward. The petroleum industry standardizes all its measurements to 60 °F (15.6 °C) specifically to avoid this problem, applying correction tables to convert any measured density back to the reference temperature.

The rate of expansion varies by oil type. Lighter oils expand more per degree of temperature change than heavier ones. Research on biodiesel, a fuel derived from plant or animal fats, found thermal expansion coefficients around 8.4 × 10⁻⁴ per degree Celsius. In practical terms, that means a 10 °C rise in temperature increases the volume of the fuel by about 8 liters per thousand liters.¹ Crude oils behave similarly, though the exact coefficient depends on composition. For a refinery receiving tanker loads of crude, that kind of volume shift translates into real money: a cargo measured at port in the tropics will read as a slightly different volume when it arrives in a cooler climate, even though the mass has not changed at all.

Work on crude oil systems under reservoir conditions confirms that density is more sensitive to temperature changes than to pressure changes in many situations.² This matters underground, where temperatures can exceed 200 °F and pressures can reach thousands of pounds per square inch, because engineers need accurate density values to predict how oil will flow through rock and up through wellbores.

What Happens to Density Underground

Crude oil in a reservoir is not the same fluid that arrives at the surface. Deep underground, the oil is saturated with dissolved gases, primarily methane and other light hydrocarbons, held in solution by the enormous pressure. Those dissolved gases lower the oil’s density substantially. A crude that measures 30° API at the surface might behave like a much lighter fluid at reservoir depth because it has so much gas dissolved in it. As the oil rises through the well and pressure drops, those gases come out of solution (a process called “gas breakout”), and the oil gets denser and heavier.

Research on CO₂-crude oil systems has shown that injecting carbon dioxide into a reservoir also lowers the density of the mixed fluid, up to a point. Under certain conditions, higher CO₂ content causes density to decrease, while higher water content pushes it back up.² Engineers exploiting these effects use CO₂ injection as a technique to make heavy oil flow more easily toward production wells. The interplay of temperature, pressure, gas content, and water cut makes reservoir fluid density a moving target, which is why downhole sensors that measure density in real time are so valuable in modern oil production.

Why Oil Density Matters in a Spill

When oil hits water, whether it floats or sinks depends almost entirely on relative density. Most fresh crude oils are lighter than both saltwater and freshwater, so they initially form a slick on the surface. That is actually helpful from a cleanup standpoint, because surface slicks can be contained with booms, skimmed, or burned. The nightmare scenario for responders is oil that sinks, because recovering submerged oil from a lakebed or seafloor is vastly more difficult.

Heavy crudes and diluted bitumen (dilbit) are the products most likely to cross that sinking threshold. Dilbit is a blend of extremely heavy bitumen mixed with lighter hydrocarbons to make it fluid enough to pump through pipelines. When dilbit is spilled on water, the lighter components evaporate quickly, and the remaining residue gets progressively denser. Research simulating dilbit spills in boreal lake conditions found that as volatile hydrocarbons left the slick, the density of the remaining oil rose above 1 g/mL, exceeding the density of freshwater. In those experiments, the oil began sinking to the bottom within about 12 days for smaller spills and around 31 days for larger ones, with wind and rain accelerating the process by breaking up the surface slick.³

Marine environments add another layer of complexity. When spilled oil interacts with sediment particles, it can form clumps called oil-particulate aggregates. Studies of dilbit in saltwater have shown that fresh to moderately weathered oil mixed with fine and medium sediments forms these aggregates and sinks, while very heavily weathered oil instead forms discrete free-floating tarballs.⁴ The relationship between density and buoyancy in a real spill is therefore not as simple as checking whether the oil is lighter than water at the moment it goes overboard. You have to account for how that density will change over hours and days as the oil weathers and interacts with its environment.

How Oil Density Gets Measured

The oldest and simplest tool is the hydrometer, a weighted glass tube that floats at different levels depending on the density of the liquid it is dropped into. You read the density (or API gravity) off the scale printed on the stem, at the point where the liquid surface intersects it. Hydrometers are cheap, reliable, and still widely used at tank farms and refineries. The standard method for measuring crude oil density by hydrometer is codified by ASTM International, and it requires the sample to be brought to 60 °F or corrected to that temperature using published tables.

For higher precision, especially in custody-transfer situations where millions of dollars ride on small differences in measured volume and density, the industry uses oscillating-tube density meters. These work by pushing a small sample of oil through a U-shaped tube and measuring the frequency at which the tube vibrates. Denser fluids slow the vibration, and the relationship between frequency and density is extremely precise. Digital density meters can resolve differences as small as a few ten-thousandths of a gram per milliliter, which matters when you are accounting for a supertanker’s cargo.

Inline density measurement has also become standard in pipeline operations. Coriolis flow meters, which measure mass flow rate by detecting how a vibrating tube twists when fluid flows through it, can simultaneously report density. This allows operators to detect changes in the oil stream in real time, catching things like unexpected water slugs or changes in crude blend composition before they cause problems downstream.

Converting Between Density, Specific Gravity, and API Gravity

If you encounter oil density expressed in different units, it helps to know how to move between them. Specific gravity is the simplest starting point: it is the ratio of the oil’s density to the density of water, both measured at the same temperature (usually 60 °F). Since water’s density at that temperature is very close to 1.0 g/mL, the specific gravity of a crude oil is numerically almost the same as its density in grams per milliliter. A crude with a specific gravity of 0.85 has a density of about 0.85 g/mL.

To get from specific gravity to API gravity, the industry uses a conversion where 10° API equals the specific gravity of water (1.0). A specific gravity of 0.85 corresponds to roughly 35° API. A specific gravity of 0.93 gives you about 20° API. The relationship is not linear, so you cannot simply add or subtract a fixed number to move between the two scales. But the direction is consistent: as specific gravity goes up (heavier oil), API gravity goes down.

If you need the weight per gallon from a known API gravity, you can work backward. Convert the API to specific gravity, multiply by the weight of a gallon of water (8.34 pounds), and you have your answer. For a 35° API crude: specific gravity is about 0.85, multiplied by 8.34 gives roughly 7.09 pounds per gallon. For a 20° API crude: specific gravity is about 0.93, giving about 7.76 pounds per gallon. That arithmetic is what sits behind the weight ranges listed earlier in this article.

When “Oil” Means Something Else Entirely

People searching for oil weight are not always thinking about crude petroleum. Cooking oils, essential oils, lubricants, and heating oil all have their own densities, and the differences can matter in surprisingly practical ways.

Cooking oils cluster in a fairly narrow density band. Canola, sunflower, soybean, and corn oils all hover around 0.91 to 0.93 g/mL, which works out to about 7.6 to 7.7 pounds per gallon. Coconut oil is a slight outlier because it is solid at room temperature and has a density of about 0.92 g/mL when melted. If a recipe calls for oil by weight rather than volume, the difference between varieties is small enough that substituting one for another rarely matters from a density standpoint (though flavor and smoke point are different stories).

Heating oil, which is essentially the same as diesel fuel, runs about 7.0 to 7.1 pounds per gallon. This number matters for homeowners estimating the weight of a full oil tank. A standard 275-gallon residential heating oil tank, when full, holds roughly 245 to 250 gallons (they are not filled to the brim), weighing around 1,700 to 1,775 pounds of fuel alone, plus the weight of the tank itself. That is a meaningful structural load if the tank sits on a basement floor or an elevated platform.

Synthetic motor oils and conventional motor oils have very similar densities, generally in the 0.85 to 0.89 g/mL range, but synthetic formulations can vary more widely because manufacturers adjust the base stock and additive package. For most practical purposes, a quart of motor oil weighs a little under two pounds, and five quarts for a typical oil change total around nine to ten pounds.

The Molecular Roots of Oil Density

At the molecular level, what makes one oil heavier than another comes down to two things: how tightly the molecules pack together and how heavy the individual molecules are. Paraffins (straight-chain hydrocarbons) are relatively light and pack loosely, which is why paraffinic crudes tend to be lighter. Naphthenes (ring-shaped hydrocarbons) pack more tightly and are somewhat denser. Aromatics (molecules with benzene-ring structures) are denser still. And asphaltenes, the enormous multi-ring molecules that give heavy crude its dark color and sticky consistency, are the densest organic components in petroleum.

Research examining crude oils from different geological settings has shown that density tracks closely with molecular weight distribution and the proportions of these hydrocarbon families. Studies of North Sea paraffinic oils, for example, demonstrated that you can extrapolate density across a wide boiling range if you know the composition of the lighter fractions, because the heavier fractions follow predictable trends.⁵ This is useful for reservoir engineers who need density estimates for fractions they cannot easily sample directly.

The geological history of a crude oil also shapes its density. Oils formed from marine organic matter tend to be different in composition from those formed from lacustrine (lake) or terrestrial sources. Oils that have spent more time at higher underground temperatures tend to be lighter, because heat breaks large molecules into smaller ones over geological time. Conversely, oils that have been biodegraded by bacteria near the surface tend to be heavier, because the bacteria preferentially consume the lighter molecules and leave the heavy ones behind. This is why tar sands and oil sands deposits, which have experienced extensive biodegradation, contain some of the densest petroleum on the planet.