API Gravity Calculator
Enter a density, specific gravity, or API gravity reading and the calculator converts all three in real time, classifies the petroleum product by its industry grade, and walks you through every step of the math. Pick a preset for common petroleum products or enter a custom density in any of four units.
Formula
Worked example
A medium crude oil sample has a density of 875 kg/m3 at 60 degrees F. Step 1: SG = 875 / 999.016 = 0.8759. Step 2: API = 141.5 / 0.8759 - 131.5 = 29.95 degrees API. This falls in the medium crude range (22.3 to 31.1 degrees API), so the liquid floats on water (SG < 1). Reversing: SG = 141.5 / (29.95 + 131.5) = 0.8759.
What is API gravity?
API gravity is a measure of how heavy or light a petroleum liquid is compared to water. It was defined by the American Petroleum Institute (API) and adopted as an industry-wide standard for crude oil and refined product trading. The scale is intentionally inverted: the higher the API gravity, the lighter the liquid. Water has an API gravity of exactly 10 degrees, so any petroleum product above 10 degrees floats on water, and anything below 10 degrees sinks. This counterintuitive direction means that expensive, highly refined products like gasoline (55-65 degrees API) sit at the top of the scale, while dense, unprocessed heavy crude and bitumen (below 10 degrees) sit at the bottom.
API gravity formula and how to use it
The defining equation is API = 141.5 / SG - 131.5, where SG is the specific gravity of the liquid measured against water at exactly 60 degrees F (15.56 degrees C). To go the other way, SG = 141.5 / (API + 131.5). Because specific gravity is density-relative-to-water, you can also start from an absolute density: SG = density of liquid / density of water at 60 F, where water has a density of 999.016 kg/m3 at that reference temperature. This calculator accepts density in kg/m3, g/cm3, pounds per US gallon, or pounds per cubic foot, and converts to SI first before applying the formula. All measurements must be at the 60 F reference temperature; if your sample was measured at a different temperature, apply an ASTM volume correction factor (VCF) to normalise it first.
Crude oil grades and what they mean for refining
The petroleum industry classifies crude oil into four broad grades by API gravity, and these grades have a direct effect on refinery economics. Extra-light products (above 45 degrees API) include condensate and natural gasoline recovered at the wellhead, which need minimal processing. Light crude (31.1 to 45 degrees API) is the most sought-after grade: it yields the highest proportion of petrol, jet fuel, and diesel with the least refinery energy per barrel. Medium crude (22.3 to 31.1 degrees API) requires more hydrotreating and catalytic cracking to extract the same distillate volumes. Heavy crude (10 to 22.3 degrees API) needs coking or hydrocracking units and tends to carry a price discount to light benchmarks. Extra-heavy crude and bitumen (below 10 degrees API) cannot usually flow to the surface unaided; it requires steam-assisted gravity drainage (SAGD), diluent blending (dilbit), or upgrading to a synthetic crude before pipeline transport. The price differential between heavy and light crude can reach 20 to 30 USD per barrel at times of tight refinery coking capacity.
Specific gravity, density, and unit conversions
Specific gravity (SG) is a dimensionless number: it is simply the ratio of a liquid's density to the density of water at the same reference temperature. For petroleum, that reference is always 60 degrees F. Because specific gravity is dimensionless, the API formula works regardless of which density unit you start with, as long as you use the same water density reference. Common density units in the petroleum industry are kg/m3 (used internationally and in pipeline operations), g/cm3 (equivalent to specific gravity numerically, but with units), pounds per US gallon (used in American field operations and product pricing), and pounds per cubic foot (used in some engineering calculations). The conversions are: 1 g/cm3 = 1000 kg/m3; 1 kg/m3 = 1/119.826 lb/US gal; 1 kg/m3 = 1/16.0185 lb/ft3.
API gravity classification for crude oil and petroleum products
| Product / grade | Typical API (degrees) | Typical SG | Typical density (kg/m3) | Float on water? |
|---|---|---|---|---|
| Gasoline | 55-65 | 0.720-0.748 | 720-748 | Yes |
| Jet fuel (Jet-A) | 40-50 | 0.775-0.830 | 775-830 | Yes |
| Kerosene | 40-55 | 0.748-0.820 | 748-820 | Yes |
| Diesel (No. 2) | 30-45 | 0.820-0.876 | 820-876 | Yes |
| Gas oil | 26-35 | 0.850-0.900 | 850-900 | Yes |
| Light crude oil (>31.1) | >31.1 | <0.870 | <870 | Yes |
| Medium crude (22.3-31.1) | 22.3-31.1 | 0.870-0.920 | 870-920 | Yes |
| Heavy crude (10-22.3) | 10-22.3 | 0.920-1.000 | 920-1000 | Yes / borderline |
| Extra heavy / bitumen (<10) | <10 | >1.000 | >1000 | No - sinks |
| Water (reference) | 10 | 1.000 | 999 | Reference |
Reference values at 60 degrees F (15.56 degrees C). Specific gravity relative to water (SG = 1.000).
Frequently asked questions
What does a higher API gravity number mean?
A higher API gravity means a lighter, less dense liquid. Gasoline has an API gravity of roughly 55-65 degrees, while heavy crude may be only 15-20 degrees. The scale is inverted by design: water sits at 10 degrees API, and every point above 10 means the liquid floats on water.
At what temperature should API gravity be measured?
The API standard specifies 60 degrees Fahrenheit (15.56 degrees Celsius) as the reference temperature. Density changes with temperature, so if you measure at a different temperature you must apply an ASTM D1250 volume correction factor (VCF) to convert the observed gravity to the standard 60 F basis before using the formula here.
What is the API gravity of water?
By definition, water has an API gravity of exactly 10 degrees. Liquids with an API gravity above 10 degrees are lighter than water and float; liquids with an API gravity below 10 degrees are heavier than water and sink. Bitumen typically sits at 8-9 degrees API and will sink in fresh water.
What is the difference between API gravity and specific gravity?
Specific gravity (SG) is the direct ratio of a liquid's density to water's density at 60 F (a dimensionless number where water = 1.000). API gravity is a non-linear transformation of that ratio using the formula API = 141.5 / SG - 131.5. The two values carry the same information; API gravity is simply the historical scale that the petroleum industry standardised on, and it compresses the useful part of the SG range into a more readable whole-number spread.
Why does crude oil price depend on API gravity?
Refineries make money by converting crude into products like petrol, jet fuel, and diesel. Light crudes (high API) yield more of these valuable distillates per barrel with less energy and fewer processing units, so they command a price premium. Heavy crudes require coking, hydrocracking, and more catalyst, all of which add cost. The Brent and WTI benchmarks are both light crudes (roughly 38 and 40 degrees API respectively), and heavier grades trade at discounts of several dollars to more than 20 dollars per barrel depending on refinery capacity.
Can I calculate API gravity from specific gravity directly?
Yes. Use API = 141.5 / SG - 131.5. For example, a liquid with SG = 0.876 gives API = 141.5 / 0.876 - 131.5 = 30.0 degrees API. This calculator's "Solve for API from SG" mode does this in one step. The reverse is SG = 141.5 / (API + 131.5).
What API gravity is considered light crude?
The industry threshold most commonly used is 31.1 degrees API or higher. Some classifications further divide light crude into "light" (31.1 to 45 degrees) and "extra light" or "condensate" (above 45 degrees). The major benchmarks WTI and Brent both qualify as light crudes at approximately 40 and 38 degrees API respectively.