The title says it all. How can I gauge how long my novel would be, if for example I've written 400 single-spaced pages of size 12 Times New Romans in Microsoft Word?
I cheated once: I pulled a book off my shelf and recreated it (in InDesign, but you might be able to do it in Word). Page size, margins (I used a ruler), font, type size, everything. Once I recreated what was on the page, I had a gauge for size. Then I dumped my current work into that, and I had a rough idea of how long my "novel" was.
The standard way of calculating word count, aside from simply using the "word count" feature of your word processor, is to format your document in standard submission format and then multiply the number of pages by 250. This is the technique that was used back in the days before computers could instantaneously count the number of words in a document. It also has the incidental advantage of accounting for the actual length of your paragraphs on the page, so that long passages of dialogue (which consist of lots of short paragraphs) or lists are correctly adjusted for.
It's almost impossible to be able to calculate this without knowing the final format the printed work will be in. There are a huge number of variables that will all have an impact on the length of the text -
The font chosen, the type size, the linefeed (font leading), the paragraph spacing, the hyphenation settings, whether the text is justified or unjustified, the kerning, the letter and word spacing, the text column width, the text column depth, the likely number of footnotes and so on.
Without really knowing these parameters any estimation will only be a very rough one which could be out by as much as 40-50%.
You can set a page size in Word. Don't use 8.5 x 11, use whatever the real, final page size will be. Also set the correct margins. If you're self-publishing and producing your own master, you can produce the PDF or whatever format directly from this and you'll know exactly what will be on every page. But even in general, it will at least give you a rough idea of how much fits on each page and how many pages the final book will be.
I did my second book with the intended final page size and margins in Word and I found that helpful to give me a feel for how long chapters would look, as well as to help me keep toward my targetted total length. In my case I figured that 250-300 pages was a good length for that particular book and I had it divided into five sections so I wanted at least roughly 50 pages each. So as I was working I had a feel for whether I was beeing too wordy or not wordy enough, whether I needed more material or had to trim, etc.
If you are using Word to create your manuscript, then you can use Word Count to find its length and work from there.
I would expect typical text (whatever that is) set in 12 point Times New Roman to contain approximately 600 words (4000 characters) per page. So a 400 page manuscript would be about 240,000 words.
That will be a big book! The Historian by Elizabeth Kostova is around that size and it weighs in at 900 pages in paperback.
Have you considered whether your magnum opus would "work" in two volumes?
I went ahead and broke it down: Give or take, the average paperback has 250-350 words per page. Thats 35 lines at 10 words per line. Of course some of those lines may only have 3 or 4 words (for dialogue). So again we're at, say, 300 words. take your total word count and divide it by 300. that'll give you an idea, not accurate down to the letter but still a pretty good idea. I used James Pattersons "3rd degree" and Dean Koontz's "Odd Hours" for reference as they were closest to me atm.