There are lots of ways to solve unit conversion problems, but I thought I would share my favourite little program. Convert for windows. It's simple, it's free, it's a tiny little tool, it has a good range of unit types covered. It'll take you 1 minute to try it.
Another good option online, when you have a really involved conversion problem, is Wolfram Alpha. I just did a search on "32 lb/ft3 in kg/m3" and it solved it in one step. Nice.