Density

Density is the amount of something per unit volume. Most typically, one expresses the mass per unit volume for a solid or liquid.

One of the most common uses of density is in how different materials interact when mixed together. Wood floats in water because it has a lower density, while an anchor sinks because the metal has a higher density. Helium balloons float because the density of the helium is lower than the density of the air.

Density and specific gravity have very similar, but not quite identical definitions.

Specific gravity

Specific gravity is a ratio of the mass of a material to the mass of an equal volume of water at 4 °C (39 °F). Because specific gravity is a ratio, it is a unitless quantity. For example, the specific gravity of water at 4 °C is 1.0 while its density is 1.0 g/cm3.

API Gravity

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy a petroleum liquid is compared to water.

If the API gravity of a petroleum liquid is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks.

API gravity is thus an inverse measure of a petroleum liquid’s density relative to that of water (also known as specific gravity). It is used to compare densities of petroleum liquids. For example, if one petroleum liquid is less dense than another, it has a greater API gravity. Although API gravity is mathematically a dimensionless quantity (see the formula below), it is referred to as being in ‚degrees‘.

The formula to convert Specific Gravity (SG) into API Gravity is:

API  gravity = 141.5/SG – 131.5