Specific Gravity formula is
Specific Gravity = Density of Body/Density of Body at 4oC
Now
Density = Mass/Volume
Density = M/L3
So Dimensional Formula of Specific Gravity= M0L0T0
Note: Specific Gravity is defined as change in Density of Body over Density of Body at 4oC. Now we know that both Density of Body and Density of Body at 4oC has same dimensions i.e. Mass/Volume so same units cancel each other leaving numeric value only. So Specific Gravity has no unit.