The way I've figured that out is by good old-fashioned counting. Take a novel, open to random pages, and count the number of words on the page. Take a rough average of about 10 pages with the same volume of words on them (like don't count half-pages at the end of a chapter, for example), and that's the target number of words you want per page when you write.
From my own notes:
The average novel has 400 words per page.
The average word-processed page in Microsoft Word has just under 600 words per page, single-spaced, and just over 300 words per page, double-spaced.
The average hand-written notebook page has between 200-300 words per page, depending on the size of the page.
From there, it's just a numbers game. If you want 15 novel pages, you would need 15 pages x 400 words per page, and that comes to 6,000 words per chapter. At just under 600 words per page, you would need a little more than ten Microsoft Word pages, single-spaced (or just under 20 pages, double-spaced) to equal 15 novel pages.
Of course, this is all rough estimates based on a a solidly-filled page, but it works pretty well to give you an idea.
