Mass density says how much mass belongs to a unit of occupied volume. The three calculator modes are algebraic rearrangements of that same statement, not separate physical laws.
Divide mass by volume
For density, both measurements must describe the same amount of material. One kilogram occupying one litre gives 1 kg/L, exactly 1000 kg/m³ and 1 g/cm³.
Multiply density by volume
When density is supplied, multiplying it by the matching volume cancels the volume unit and leaves mass. Convert both quantities to coherent base units before multiplying.
Divide mass by density
Mass divided by mass per volume leaves volume. DensityCalc performs that cancellation with exact rational factors, then displays cubic metres, litres, cubic centimetres, or cubic feet.