Medical Math Question...?

I can't remember when converting how do you know if you should divide or multiply....like if I was given 24,000,000 mcg and they needed to be in g or if I had 100 L that needed to be in mcL.....Is there a rule that says bigger to smaller do this and smaller to bigger to that...something that would apply across the board...Thanks so much school starts on Monday and I am trying to study dosage calculations without my notes....