I don't know how to do that exactly, but I do have some articles that can help you estimate:
http://www.sfwa.org/writing/faq2.htm and scroll down to "How do you decide where the novella ends and the novel starts?" to get a rough idea as to whether your book is a huge or small novel, or whether your short story is 40 pages (which I would assume is around 7500 words, based on their answer) or 15.
For word counts by publishing standards: http://www.sfwa.org/writing/wordcount.htm