While it's not possible to take into all these factors when estimating how many words per page there will be for your writing, this calculator can give a more accurate words per page conversion estimate than the typical 250/500 ballpark figure. The truth is the number of words per page depends on a variety of factors such as the type of font, the font size, spacing elements, margin spacing, and paragraph length to name a few.
While a general guideline is one page is 500 words (single spaced) or 250 words (double spaced), this is a ballpark figure. There are times when it helps to know how many words per page you're writing.