About API Gravity
API gravity is the scale the American Petroleum Institute defined to express how light or heavy a petroleum liquid is compared with water. It is derived directly from specific gravity (SG), the ratio of the liquid's density to the density of water, both measured at the industry-standard reference temperature of 60°F. The formula, standardized in ASTM D287, is:
°API = (141.5 / SG) − 131.5
Why the scale is built this way
- Inverse relationship: because SG sits in the denominator, a lower specific gravity (lighter liquid) produces a higher °API, and a higher specific gravity (denser liquid) produces a lower °API.
- Water as the anchor point: pure water has SG = 1.000, which the formula converts to exactly 10°API. Anything above 10°API is less dense than water and floats; anything below 10°API is denser and sinks.
- Industry classification: crude oil is grouped by °API into light (> 31.1), medium (22.3–31.1), heavy (10–22.3), and extra heavy (< 10). Lighter crude typically refines into a larger share of gasoline and diesel and trades at a premium over heavier grades.
Converting to density and weight
Once specific gravity is known, density in metric units follows from SG × 999.016 kg/m³, the density of water at 60°F. For bulk cargo and custody-transfer estimates, the petroleum industry uses the convention that a US barrel (42 gallons) of water at 60°F weighs about 350.15 pounds; multiplying barrels by SG and by 350.15 gives an estimated total weight in pounds, which this calculator also converts to US short tons.
Getting accurate results
- Specific gravity should be measured, or already referenced, at 60°F/60°F — the same liquid measured at a different temperature will read differently, since petroleum liquids expand when heated.
- Most crude oils fall between roughly 0.80 and 1.00 SG (about 10–45°API); values far outside that band usually mean a data-entry or unit error.
- The weight conversion assumes standard US barrels and is an estimate for bulk liquid — it does not account for temperature correction tables (ASTM D1250) used in precise custody-transfer accounting.