If you do an internet search for 'genealogy relationship calculator' you can find some slightly hard to use sites that want you to fill in blanks, but here are some I like:
This one explains some of the important terminology--http://www.searchforancestors.com/utility/cousincalculator.h...
This one has a chart that is easy to use:http://genealogy.about.com/library/nrelationshipchart.htm
Note that you have to already know who your common ancestor is!
Good hunting :D