How to determine the strength of class coupling?

How can I determine the strength of the coupling between two classes? Is there a way to order the couplings by their relative strength?

For example, let classes A and B be abstract and the class C be a regular class. Then are the classes A and B in the expression A extends B coupled stronger together than the classes A and C in the expression A extends C? Are the classes A and B in the expression A instantiates a child of B coupled stronger together than the classes A and C in the expression A instantiates C?

Has anyone found a way to formalize this?