Think about it in terms of the units and it makes much more sense, for example:
Density in kg/m3
Volume in m3
How do you turn kg/m3 into kg? Multiply by m3, thus to obtain volume in kg:
vol = density * volume
In calculations whatever you do to numbers you must also do to the units and vice-versa, so if you're ever stuck then look at the units involved and work it out from there.
HTH,
--rob