publishing - Is it possible to establish a relation between word count and page count?

I found some nice replies dealing with word counting, but my question is slightly different.

I want to know if it's possible to establish a relation between word count and the number of pages in an average formatted book, so I can have an idea of how many pages my manuscript probably has so far.

Of course this should be just an average, not an exact answer. Basically, I want to know, in average, how many words a page has.