How this instrument works
Degrees API say how light a petroleum liquid is, and they count backwards — a higher number means a less dense oil. Specific gravity compares a sample's density against water at 60 °F, so water itself sits at exactly 10 °API. West Texas Intermediate lands near 39.6, Brent near 38.3, and Canadian bitumen down around 8, low enough that it sinks in water instead of floating.
Refiners inherited these constants by accident. Nineteenth-century hydrometers used Baumé's formula, 140/SG − 130, but in 1921 America's National Bureau of Standards found that US instrument makers had been etching stems to a modulus of 141.5 rather than 140. Recalling every hydrometer in every refinery was unthinkable, so American Petroleum Institute blessed what was already engraved on glass and published 141.5/SG − 131.5. Two odd constants: a manufacturing error made permanent.
Two conditions hold this definition together, and both get quietly dropped. First, both densities belong at 60 °F (15.56 °C) — a barrel measured hot reads lighter than it truly is, which is why cargo surveyors correct volumes to 60 °F before quoting anyone a number. Second, an inverse relation is not a linear one: moving from 20 to 25 °API shifts density far more than moving from 50 to 55 does. Push specific gravity toward zero and degrees API race off to infinity; push it upward and they crowd against a floor of −131.5. No real hydrocarbon visits either limit.
- Enter your measured figure in Specific gravity at 60 °F — a dimensionless ratio against water, usually 0.70 to 1.05 for crude.
- Check that your lab reported density at 60 °F rather than at ambient temperature, and correct it first if not.
- Read API gravity (degrees) below. Above 31 trades as light crude, below 22 as heavy, under 10 as extra-heavy.
- Going backwards? Invert with SG = 141.5 ⁄ (°API + 131.5), or try values in Specific gravity at 60 °F until output matches.
Worked example — checking a hydrometer in water
Before a cargo surveyor trusts a glass hydrometer, they float it in distilled water held at 60 °F. Specific gravity at 60 °F reads 1. Feed that in: 141.5 ⁄ 1 = 141.5, subtract 131.5, and API gravity (degrees) returns exactly 10.
That 10 is no coincidence — it is where an entire scale hangs. Every other graduation etched up a hydrometer stem was laid out from this one anchor, so an instrument missing 10 in pure water will misread every crude it ever touches. It also fixes a useful boundary: above 10 °API a sample floats, below 10 it sinks.
Questions
Why does heavier oil get a lower API number?
Because specific gravity sits in a denominator. Dense oil means large SG, small quotient, and less left over once 131.5 is subtracted. Bitumen at SG 1.02 scores about 8.7 °API; gasoline at SG 0.74 scores about 60. Sorting a list by degrees API sorts it heaviest to lightest — exactly opposite to sorting by density, which trips up newcomers constantly.
Are API degrees a temperature?
No. Nothing thermal is involved. Here 'degrees' merely labels positions along an arbitrary hydrometer scale, much like degrees Baumé or degrees Brix. Only temperature in play is 60 °F, and that is a fixed reference condition for measurement, not a result. SI lists no unit for API gravity because it is a defined trade scale rather than a physical quantity.
What if I measure at 25 °C instead of 60 °F?
Your answer drifts light. Liquid hydrocarbons expand roughly 0.07% per °C, so a sample read at 25 °C (77 °F) looks less dense than it would at 15.56 °C and reports perhaps 0.5 to 1 °API too high. Petroleum laboratories correct readings to 60 °F using standard volume-correction tables before converting. On a 500,000-barrel cargo priced by grade, that correction moves real money.
How do light, medium and heavy crude divide up?
Common industry cut-offs put light crude above 31.1 °API, medium between 22.3 and 31.1, heavy below 22.3, and extra-heavy grades or bitumen under 10. Boundaries vary by agency and by contract, so treat them as trade shorthand rather than physics. Refiners care because lighter feedstock yields more gasoline and diesel per barrel with far less upgrading.
Can I convert degrees API into kg/m³?
Yes, in two steps. Invert to specific gravity with SG = 141.5 ⁄ (°API + 131.5), then multiply by density of water at 60 °F, about 999.0 kg/m³. A 35 °API crude gives SG 0.8498 and roughly 849 kg/m³. Multiplying by 1000 instead is a common shortcut costing about 0.1% — usually tolerable, occasionally not.
Why 141.5 and 131.5 rather than round numbers?
They are fossils of a 1921 correction. Baumé's older scale used 140 and 130, but American hydrometers had been calibrated to 141.5 by mistake. Picking new constants that matched existing glassware kept water at exactly 10 and spared an industry from replacing its entire measuring stock. Convenience beat elegance, and 141.5 has stood ever since.