What is the difference between density and specific gravity? (FAQ - Mass & Density)
Density is defined as mass per unit volume; it has the SI unit kg·m-3 and is an absolute quantity. Specific gravity is the ratio of a material's density with that of water at 4 °C (where it is most dense and is taken to have the value (*) 999.974 kg·m-3) and is therefore a relative quantity with no units.
(*) The words taken to have the value are important here as the value of maximum density cannot be known exactly (nor, for that matter, can the temperature of 4 °C be realised exactly); the problem is analogous to one involving the so-called manometric units in pressure metrology.)