API Gravity Calculator

Why the oil industry measures density backwards

API gravity is density expressed on an inverted, stretched scale. A light condensate might read 55 °API and a heavy Venezuelan crude 9 °API, and the higher number is the lighter fluid. The scale was adopted by the American Petroleum Institute so that hydrometer readings on a linear stem would spread the commercially interesting range of crude oils across a wide span of numbers rather than crowding them between 0.80 and 1.00 specific gravity.

The two constants are not arbitrary. API = 141.5 ÷ SG − 131.5 is constructed so that SG = 1.000 gives exactly 10 °API. That single anchor is worth remembering, because it tells you instantly whether an oil floats: above 10 °API it is lighter than fresh water, below 10 it sinks.

The reason gravity matters commercially is that it correlates with what a refinery can make. Lighter crudes yield more naphtha, gasoline and middle distillate per barrel and less residue, so they normally command a premium over heavy crudes of similar sulphur content. Gravity is one of the two headline numbers in every crude assay — the other is sulphur — and together they place a crude in the sweet/sour and light/heavy quadrants that price differentials are quoted against.

Gravity is also how the industry moves between the two units it trades in. Oil is bought by volume in barrels in the Americas and by mass in tonnes almost everywhere else, and the conversion between them is nothing more than density. That conversion is why a scheduler cares about the third decimal place.

The four conversions, and the 60 °F basis behind all of them

API to specific gravity. Rearranging the definition gives SG = 141.5 ÷ (API + 131.5). Both forms assume the 60/60 °F basis: oil at 60 °F compared with water at 60 °F. Quote a gravity without a temperature and you have quoted nothing, because oil expands roughly half a percent for every 10 °F.

Specific gravity to density. Multiply by the density of water at 60 °F, 999.016 kg/m³. Note this is not 1,000 — water is densest at about 39 °F and has already expanded slightly by 60 °F. Using 1,000 introduces a consistent 0.1% error, which is invisible in a lab report and material on a 700,000 barrel cargo.

Density to pounds per gallon. One US gallon is 0.0037854118 m³, and one kilogram is 2.20462 lb, so lb/gal = density × 0.0037854118 × 2.20462. Water at 60 °F comes out at 8.3372 lb/gal, and a 32 °API crude at 7.2154 lb/gal. Truck and rail loading calculations run in these units.

Density to barrels per tonne. A tonne occupies 1,000 ÷ density cubic metres, and one cubic metre is 6.2898108 barrels, so bbl/t = 6,289.8108 ÷ density. A 32 °API crude gives 7.27489 bbl per tonne. This is the number that turns a cargo quoted in tonnes into a volume you can schedule into tankage.

Temperature correction. Volumes measured at any temperature other than 60 °F must be corrected before they can be compared or invoiced. The API approach computes a thermal expansion coefficient from the 60 °F density, α = K₀ ÷ ρ₆₀², using the crude-oil group value K₀ = 341.0957 with K₁ = 0, then applies VCF = exp[−α·ΔT·(1 + 0.8·α·ΔT)]. Multiply gross observed volume by the VCF to get net standard volume. Because API gravity is already defined at 60 °F, no iteration is needed here — the density the coefficient needs is the one you started from.

Reading the gravity number

The conventional bands are light above 31.1 °API, medium from 22.3 to 31.1, heavy below 22.3, and extra heavy below 10 where the oil is denser than water. These boundaries are industry convention rather than a standard with the force of a specification, and individual pipelines, refineries and price benchmarks all draw their own lines, so treat them as orientation rather than as a rule.

What the bands really encode is transportability and yield. Above about 30 °API a crude flows in a pipeline at ambient temperature and yields a high proportion of light products. Between 20 and 30 it usually still flows but the yield slate is heavier. Below about 20 the viscosity rises sharply and the crude needs heating, diluent or upgrading to move at all — which is why extra-heavy production is typically blended with condensate to reach a pipeline specification gravity rather than shipped neat.

Do not read gravity as a proxy for quality on its own. Sulphur content, acidity, metals and pour point all matter, and a light sour crude can be worth less than a medium sweet one at the same refinery. Gravity tells you about density and, loosely, about the yield slate; it says nothing about what has to be taken out.

Watch the temperature basis whenever you compare figures from different sources. API gravity is defined at 60 °F, which is 15.56 °C, while many laboratories outside North America report density at 15 °C. The two differ by about 0.5 kg/m³ for a typical crude — small, but enough to move an API gravity in the second decimal place, and enough to matter when a contract has a gravity-based price adjustment.

API gravity, specific gravity, density and volumetric conversions

Every row is produced by running the same conversions this calculator uses: SG = 141.5 ÷ (API + 131.5), density = SG × 999.016, lb/gal = density × 0.0037854118 × 2.20462, bbl/t = 6,289.8108 ÷ density. The 22.3 and 31.1 rows are the conventional heavy/medium and medium/light boundaries.

Assumptions, limits and traps

• Gravity without a temperature is meaningless. API gravity is defined at 60 °F. A hydrometer reading taken at tank temperature must be corrected before it becomes an API gravity.
• 60 °F is not 15 °C. It is 15.56 °C, and densities reported on the two bases differ by roughly 0.5 kg/m³ for a typical crude.
• Water at 60 °F is 999.016 kg/m³, not 1,000. Using 1,000 builds in a consistent 0.1% bias in every density and tonnage.
• The volume correction here uses the crude-oil group coefficient. Refined products, jet fuel, gasoline and lubricating oils each have their own coefficients, and using the crude value on a product volume is wrong.
• This is not a custody transfer calculation. Custody transfer follows the edition of API MPMS your contract names, including its rounding and implementation rules, and normally also applies a pressure correction and a sediment-and-water deduction.
• Blending is linear in density, not in API gravity. Blend two crudes and the volume-weighted density is right; the volume-weighted API gravity is not, because the scale is a reciprocal.
• Barrels per tonne is a density conversion, not a constant. A single 'about 7.3 barrels per tonne' figure is only correct for one gravity.

Where gravity feeds into the rest of the oilfield arithmetic

Density is the bridge between volume and everything else in a hydrocarbon system. In the wellbore it sets the hydrostatic column: a fluid's hydrostatic pressure is density times height times gravity, which is why mud engineers work in pounds per gallon — the same unit this calculator produces — rather than in API gravity.

In the tank farm it sets the relationship between gauged height, volume and mass, and it is what makes a pipe or vessel volume convertible into a shipment weight. In the commercial office it sets the barrels-per-tonne factor that turns a cargo priced in dollars per barrel into one priced in dollars per tonne.

On the revenue side, gravity often appears directly in the price. Many crude purchase contracts carry a gravity adjustment that adds or deducts cents per barrel per degree of API away from a reference, which flows straight through to what a mineral owner receives — the mechanics of that flow are in the oil and gas royalty calculator.

For energy content, gravity is a weak proxy and should not be used as one. Lighter crudes have more energy per unit mass and less per unit volume, so the direction of the correlation depends on which basis you are asking about; the fuel BTU content calculator handles that properly using heating values rather than density.