How is Specific Gravity defined?

Specific Gravity, SG, is a unit less quantity that provides the scientist or engineers an idea of how dense a substance is compared to water. The density of water, which is one kilogram per liter (at 4 degrees C), is assigned a SG of 1.000. If a substance is denser than water, it will have a SG greater than 1.000; if it is less dense than water, its S.G. will be a value less than 1.000 (but greater than zero). Let's say a matter has a density of 2.5 kilograms per litre. That means that its SG is 2.5 (2.5 divided by 1.000).