The API gravity calculator above moves between the three ways the density of a petroleum liquid gets quoted: degrees API, specific gravity at the 60/60 °F reference, and absolute density in kilograms per cubic metre. Enter whichever you have and the other two follow, along with the pounds per gallon and barrels per tonne figures that turn a density into a quantity.
Arb Digital publishes a general specific gravity calculator that handles the ratio of any substance to water, and a density converter that rescales density units. This page is the petroleum-specific member of that family: it owns the API scale, its 60 °F reference and the volume-to-mass arithmetic that goes with it.
What This API Gravity Calculator Does
Degrees API is a density scale invented for the oil industry, and it has two properties that make it awkward if you meet it cold. It runs backwards, so a higher number means a lighter liquid. And it is not linear in density, because it is built on the reciprocal of specific gravity, so a step of one degree API means different amounts of density change at different points on the scale.
The tool converts in all three directions and always reports all three quantities, so you can see the relationship rather than just the answer. It also produces two derived figures that matter commercially: pounds per US gallon, which is how a lot of North American product movement is quoted, and barrels per metric tonne, which is the conversion factor between the volume the industry measures in and the mass that ships and refineries account in.
Everything on this page is defined at 60 degrees Fahrenheit, which is 15.56 degrees Celsius. That reference is not decoration. Liquids expand when warm, and a barrel of crude measured at forty degrees and a barrel measured at eighty degrees are not the same amount of oil. The reference temperature is what makes a quoted gravity comparable between two parties.
How to Use It
- Choose the input you actually have. A lab report will usually give you a specific gravity or a density; a pipeline or trading document is more likely to quote degrees API.
- Check that your figure is at or corrected to 60 °F. A density measured at another temperature has to be corrected before it means anything on this scale.
- Read the specific gravity as the pivot. The API formula is defined on specific gravity, and density is derived from it, so specific gravity is the quantity everything else hangs from.
- Enter a volume if you need a mass. The tool converts barrels to tonnes at the density it has just worked out, which is the arithmetic behind any volume-to-weight ticket.
- Watch what happens below 10 °API. The tool tells you when a liquid is denser than water, which is the one hard physical threshold on the whole scale.
The Formula: How API Gravity Is Calculated
The definition is API gravity = (141.5 ÷ SG) − 131.5, where SG is the specific gravity of the liquid at 60 °F relative to water at 60 °F. Inverting it gives SG = 141.5 ÷ (°API + 131.5). The US Energy Information Administration's glossary entry for API gravity states the relation in exactly that form.
The two constants are chosen rather than derived, and they were chosen so that fresh water lands on 10 degrees API: substitute SG = 1 and you get 141.5 − 131.5 = 10. That single anchor is what makes the scale readable, because it puts the water line at a round number and gives every petroleum liquid lighter than water a value above it.
To get an absolute density the tool multiplies specific gravity by the density of water at the same reference temperature, taken as 999.016 kg/m³ at 60 °F. The measurement itself is standardised: ASTM D1298, the standard test method for density, relative density or API gravity of crude petroleum and liquid petroleum products by hydrometer, sets out the laboratory procedure and the corrections back to the reference temperature. For the underlying physics of density and specific gravity, OpenStax University Physics Volume 1, section 14.1 on fluids, density and pressure, covers the definitions.
Work the default by hand. At 35 °API, SG = 141.5 ÷ (35 + 131.5) = 141.5 ÷ 166.5 = 0.84985. Density is 0.84985 × 999.016 = 849.0 kg/m³. In imperial that is 849.0 × 0.0083454 = 7.085 pounds per US gallon. One tonne occupies 1,000 ÷ 849.0 = 1.1778 m³, and a US petroleum barrel is 0.158987 m³, so a tonne is 7.408 barrels.
Why the Scale Runs Backwards
The inversion is inherited. API gravity is a rescaling of the older Baumé hydrometer scale for liquids lighter than water, and hydrometers for that job are graduated so that a stem floating higher — a lighter liquid — reads a bigger number. The API scale kept the sense of the instrument and adjusted the constants.
The practical consequence is that the scale reads like a value scale rather than a physics one, which is roughly how the industry uses it. Lighter crudes yield more of the products refineries want most and generally need less processing, so a higher API number is normally a more valuable barrel, all else equal. The scale is a density measurement, but it is read commercially.
The non-linearity is the part that trips people. Because specific gravity sits in the denominator, a change of one degree API near the top of the scale corresponds to a smaller density change than one degree near the bottom. Averaging API gravities of two blended streams is therefore wrong; you have to convert to specific gravity, blend by volume there, and convert back. This tool makes that easy because it always shows both.
The 10-Degree Line and What Sinks
Ten degrees API is the one point on the scale that means something physical rather than conventional. It is where specific gravity equals one, so a liquid above 10 °API is less dense than fresh water and floats on it, and a liquid below 10 °API is denser and sinks.
That matters far beyond arithmetic. Spill response depends heavily on whether a product will float, and a heavy crude or bitumen near or below the water line behaves quite differently from a light crude that stays on the surface. Salinity shifts the line slightly, because seawater is denser than fresh water, so a product marginally below 10 °API can still float in the sea while sinking in a river.
The tool flags this threshold explicitly rather than leaving you to notice it. Very heavy material also breaks the practical assumptions behind hydrometer measurement, since it may be too viscous to test without heating, which then requires a correction back to the reference temperature.
Light, Medium and Heavy: Why the Bands Vary
Crude oils are routinely described as light, medium or heavy, and it is tempting to treat those as defined terms. They are not defined consistently. Different agencies, refiners, pipeline operators and sales contracts draw the lines in different places, and a barrel that is medium under one convention is light under another.
This page therefore does not publish a classification table, because a table would suggest an authority the terms do not have. If a classification matters for a transaction, the boundaries that apply are the ones written into the contract or the operator's own specification, and those are the ones to use.
Gravity is also only one dimension of quality. Sulphur content is at least as commercially significant, which is why crudes are described as sweet or sour alongside light or heavy. Acidity, metals content, pour point and the yield of individual product fractions all bear on what a barrel is worth. A single gravity number is a useful shorthand and a poor summary.
Volume, Mass and the Temperature Problem
Oil is bought and sold by volume but shipped and refined by mass, and the bridge between them is density. That is why barrels per tonne appears in the grid: it is the conversion factor that a gravity figure exists to supply.
The trap is temperature. Petroleum liquids have a large coefficient of thermal expansion, so a volume measured at ambient temperature has to be corrected to the standard reference before it can be converted or compared. That correction is the entire reason a standard reference temperature exists, and skipping it produces errors of a size that matter commercially on a cargo. Use the temperature converter to check that a quoted figure really is at 60 °F and not at 15 °C, which is a common near-miss: they differ by about half a degree.
For the surrounding arithmetic, the volume converter handles barrels, cubic metres and gallons, the mass to volume converter handles the density step directly, and the density calculator works out a density from a measured mass and volume if that is where you are starting.
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Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Averaging degrees API when blending — the scale is not linear in density. Convert to specific gravity, blend there by volume, and convert the result back.
- Using a density measured at ambient temperature — every figure on this scale is referenced to 60 °F. An uncorrected reading is not comparable with anything.
- Confusing 60 °F with 15 °C — they are about half a degree apart, and mixing the two conventions produces a small but real discrepancy in a large cargo.
- Reading the scale as a quality score — gravity is one dimension. Sulphur, acidity, metals and product yield can matter as much or more.
- Assuming a fixed barrels-per-tonne factor — it moves with density, from around 6.3 for heavy material to well above 8 for very light liquids. It has to be computed for the specific gravity in hand.
Related Free Tools From Arb Digital
For the general ratio-to-water calculation on any substance, use the specific gravity calculator. To move a density between units, use the density converter, and to work one out from a measured mass and volume use the density calculator. The volume converter handles barrels, cubic metres and gallons, the mass to volume converter handles the density step, and the temperature converter keeps the reference temperature honest. If you are working with fuels rather than crude, the fuel economy converter and the gas oil mix ratio calculator may be closer to what you need. Everything Arb Digital publishes is listed on the free online tools hub.
Frequently Asked Questions
It is a density scale for petroleum liquids defined as 141.5 divided by the specific gravity at 60 degrees Fahrenheit, minus 131.5. It runs inversely to density, so a higher number means a lighter liquid, and the constants are chosen so that fresh water reads exactly 10.
Because the constants were picked to make it so. Water has a specific gravity of one at the reference temperature, and substituting one into the formula gives 141.5 minus 131.5, which is 10. It is a chosen anchor rather than a physical result.
It is denser than fresh water and will sink in it rather than float. That is the one genuinely physical threshold on the scale, and it matters for spill behaviour. Seawater is denser than fresh water, so the practical floating line sits slightly lower at sea.
No. The scale is not linear in density because specific gravity appears in a denominator. Convert each stream to specific gravity, blend those by volume, and convert the blended specific gravity back to degrees API.
Because petroleum liquids expand appreciably when warmed, so a density or a volume only means something when it is tied to a stated temperature. Sixty degrees Fahrenheit, about 15.56 Celsius, is the convention the API scale and its test methods are built on.
It depends entirely on the density. At 35 degrees API the figure is about 7.41 barrels per metric tonne. Heavier material gives fewer barrels per tonne and lighter material gives more, so the factor has to be computed for the specific gravity you actually have.
Not automatically. Lighter crudes generally yield more of the products refineries want and need less processing, so they usually command more, but sulphur content, acidity, metals and the detailed product yield all bear on value. Gravity is a shorthand, not a full description.
This tool is provided for educational and estimating use. It applies the published API gravity definition at the 60 °F reference and does not perform volume correction from another temperature, nor does it replace a measurement made under the applicable test standard; commercial quantities and quality determinations should follow the standard, the contract and the measurement your counterparty accepts.
