I keep a copy of the publishing definitions in my head because they're concrete and handy: the Science Fiction and Fantasy Writers of America (SFWA) is often quoted — they place novellas from 17,500 to 40,000 words,
novelettes between 7,500 and 17,500, and novels at 40,000 words and up. Many traditional publishers, however, expect commercial novels nearer to 70,000–100,000 words, with exceptions by genre.
Converting to pages is a matter of math and format: if a page averages 250–300 words, a 20,000-word novella becomes about 70–80 pages and a 90,000-word novel about 300–360 pages. But typography, chapter breaks, and white space change that math quickly. Look at modern examples: '
The Great Gatsby' and '
Lord of the Flies' are slim compared to multi-hundred-page epics. For practical purposes I always check the word count first, then the projected page count for the specific trim and font, and then I decide how the story should breathe on the page. It helps me judge pacing and submission targets, and I like the way these numbers make the work feel tangible.