Books are usually categorised as novels or other formats based on the number of words that they have, rather than the number of pages. A book is usually considered to be a novel if it is 40,000 or more words long. 40,000 words is approximately 150 pages with typical formatting.
There is no strict page count that a book must have in order to be considered a novel. This is in large part because the number of pages that a book has is drastically affected by the font size, the line spacing, the paragraph spacing, the margin thickness, and the page size. One book could easily have twice the number of pages as another, even though they have the same word count, because of how they are formatted.
There is a lower limit for the word count of a novel, which is 40,000 words. However, even this is not a strict limit. This is just the typical lower limit that many literary prizes use for their novel categories.
The idea of what a novel is has changed over time; as such, defining what a novel is is very difficult. However, modern novels differ from shorter fiction formats in that they usually contain subplots. Novellas, which are usually shorter than novels at between 17,500 and 40,000 words, generally contain no subplots or just fewer subplots. Thus, the quality that really defines the difference between novels and shorter formats is the complexity of the story.
Note that while 40,000 words is the generally agreed-upon lower limit for the length of a novel, most novels are much longer than this. It is not unusual for a novel to be 70,000 to 80,000 words long, which is about 300 pages with typical formatting. In fantasy fiction, which tends to have the longest novels, it’s not unusual for books to be more than 250,000 words long.← Writing FAQs