Select your entire manuscript. The standard format is an average of 250 words to a page for most publishers.
Change the font to Courier, which is a monospaced font. This ensures each letter has the same width as the others.
Set the font size to 12. This is the standard type size for manuscripts as well as most reports.
Check each page for double spaced lines. All of your pages should be double spaced.
Make all your margins 1 inch around. This includes your top and bottom margins as well.
Pick out a page and average the word count. If you are close to 250, multiply the number of pages in your story by 250 to get your word count.