As it is the density of something divided by the density of something else, the result is a dimensionless (No unit).
Formula
It can be expressed as:
SG = ρ something / ρ H₂O
ρ something = density of the fluid (usually in kg/m³)
ρ H₂O = density of water (usually kg/m³)
It's important that both units system match so you can cancel each other.
It doesn't matter if you use SI units or the imperial english system, as long as you use them consistently. Both in SI units or both in the imperial system.
As stated above the result is a dimensionless unit that has the same absolute value regardless if you started using SI or imperial units to get it.
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