Working with Circular references in Excel
If you've come to this page, chances are you've experienced the "Circular reference warning" popping up when you opened an Excel file or entered a formula. Excel detects a circular reference when a chain of calculations visits the same cell more than once. Many users get (very) confused by this message and have no idea what it is about. I'll try to demystify that message here!
You may also find an excerpt of parts of the article on the Microsoft Office blog
The next couple of pages discuss: