How do I calculate Credit Hours?

I am working on my child's transcript and am confused about calculating credit hours.

I understand that Carnegie Credits are typically used by schools and are calculated by counting classroom time. I also have heard that depending on the difficulty of the class, more or less credits are offered. How have people worked that out for their homeschoolers?

In addition to our yearlong subjects, we have also spent quite a bit of time doing various unit studies. How should we calculate those?

Are there any books or websites that people would recommend to help?