I ran into this issue a few years back, when I wanted to enter some pieces in a contest asking for your "first page"---I carefully pared everything down to the first
manuscript page (in my case about 650 words), only to find the actual word count limit was about 250 words. (My blog post about the experience is
here, if anyone cares to read it.) I'd guess that 250 words is a quite low but reasonable estimate of a mass-market paperback page; a trade paperback or hardback page will be more.
But since you're in a word processor already: Find a book of the size you're interested in (the books CreateSpace and such produce are trade paperbacks; if you don't have a book of the size you're curious about to hand, feel free to ask someone here or elsewhere

), measure its dimensions and the margins on a typical page, then temporarily change the paper size and the margins in your Word document. The number of Word pages it takes up is a reasonable estimate.

I'm of the opinion that page counts are far inferior to word counts as a measurement; nearly any high school or college student knows various tricks for subtly increasing or decreasing page count by fiddling with margins, font size, line spacing, and the like, and from
my experience helping catalog our family's book collection I know that publishers have done much the same thing (e.g. in the war years when paper was rationed). But if you need a page count for a text you have, there's no substitute for experiment, and it's easy enough to do one. (And for writing to a page count limit ... I'd try that experiment with the output of one of those "lorem ipsum" dummy-text generators.)