Density units combine one mass unit with one volume unit. A correct conversion changes both numerator and denominator, so converting only kilograms or only cubic metres produces the wrong physical quantity.
The metric factor is exactly 1000
One gram is 0.001 kilogram, while one cubic centimetre is 0.000001 cubic metre. Dividing those factors gives 1000, so 1 g/cm³ = 1000 kg/m³ exactly.
mL and cm³ are the same volume
NIST identifies one millilitre with one cubic centimetre. Therefore g/mL and g/cm³ are exactly equivalent unit expressions, not an approximation based on water.
Pounds and cubic feet remain exact rationals
The international pound is exactly 0.45359237 kg and the international foot is exactly 0.3048 m. DensityCalc cubes the foot factor and divides exact rationals before rounding only the displayed decimal.