API gravity,

Definition of API gravity:

1. American petroleum Institutes inverted scale for denoting the lightness or heaviness of crude oils and other liquid hydrocarbons. Calibrated in API degrees (or degrees API), it is used universally to expresses a crudes relative density in an inverse measure lighter the crude, higher the API gravity, and vice versa because lighter the crude higher its market value. Oil with API greater than 30º is termed light; between 22º and 30º, medium; below 22º, heavy; and below 10º, extra heavy. Asphalt on average has an API gravity of 8°, Brent Crude of 35.5°, and gasoline of 50°. Formula: {(141.5 ÷ relative density of the crude (at 15.5°C or 60°F)} - 131.5.