In order to understand specific gravity, one must first understand what density is, since specific gravity is the ratio between density of an object divided by the density of water, both, at a specified temperature and pressure. Specific gravity is calculated at 4 degrees Celsius because, temperature affects density, and density affects specific gravity readings.
Density of water = Mass of water / Volume of water Where, mass describes the amount of matter in an object.
For example, since one cubed cm (cm3) of water at 4 degrees C = 1g, the density of water is 1g/cm3 , where g is the mass of water and cm3 is the volume of a cube that measures 1 cm × 1 cm × 1 cm. That is why the specific gravity of water at 4 degrees C is set to 1. Either g/ml or g/cm3 can be used as units to measure density.
Now that you have an idea of what density is, the formula for Specific gravity is;
Specific gravity = specific gravity of object (eg; sugar syrup) / specific gravity of water.
If a sugar syrup solution has a specific gravity greater than one, it means that it is heavier than water, and if less than one it is lighter than water.
The tool used to measure specific gravity is a hydrometer.