You may need to look deep into the calculator to know what it is doing.
SG is sometimes (usually?) defined as the ratio of the density of the liquid to the density of water at the same temperature, but this calculator defines it differently, as the ratio of the density of the substance to the density of water at 4 deg C.
In that scenario, sg = density, and I'm not sure why one even wants such a calculator. lol
This calculator will convert density to specific gravity and display a conversion scale for a range of density values centered around the entered density, SG=ρ/ρ₀