How many paper back book pages to one MS Words page?

Writers and publishers normally measure story length by word count, not page count, as there are many factors that influence how much text will fit on a page. A good rule of thumb is that one page in MS Word with the default settings equals about two pages in paperback, but it might be worth finding a book that's formatted in the way you want yours to be and counting the words on a typical page. Multiply that by 150 and you'll know how many words you need.

I believe that most people measure the length of their book not by page length (on MS) but by word count.

If I remember correctly, 12 point Times New Roman is the default size with single pacing. If you want to know how many pages (roughly) your story will be, you divide your word count by 250

Say, if your book ends up at 62,00 words, you divide that by 250 which equals = 248 pages (roughly).

This is what I have been told anyway.

The size of font in paperbacks varies a great deal. Try selecting a random three books (different publishers, different genres), count ten lines of copy and count the words per line and average them, then count the lines per page.

Keep in mind that an editor and publisher will have their own standards for publishing.

Industry standard is about 250 words per page. So take the word count of your story and divide it by 250 and you'll have your page count. (If you're using MS then it should tell you the word count in the lower left hand corner).