Percent recovery = amount of substance you actually collected / amount of substance you were supposed to collect, as a percent.

So another way of putting it:

Percent Recovery = (pure/impure) x 100

Let's say you had 10.0g of impure material and after recrystallization you collected 7.0 g of dry pure material. Then your percent recovery is 70% (7/10 x 100).

You need to make sure you material is dry or else the mass will be more than it actually is. That's why students sometimes calculate percent recoveries greater than 100% (which is impossible).

Overview of recrystallization:
www.mendelset.com/articles/680/preparation-recrystallization-acetanilide