Another method is to analyze the works of well - known authors. Take Stephen King for example. His novels vary in length, but on average, they are often in the 100,000 - 150,000 word range. By looking at a range of authors' works, you can start to build a picture of the average word count for novels as a whole.
One way is to look at best - selling novels in different genres. For example, in the mystery genre, you'll find that many popular novels are around 80,000 words. By sampling across multiple genres, you can get an idea of the general range.