Do you worry about word count when you write? I usually don’t, unless I’m entering a contest where the word limit is enforced. A long time ago, one of my fellow writers asked me what the word limit for children’s books was. I didn’t have an answer at that time. So I did some research and found these guidelines in the Writer’s Digest.
ADULT NOVELS: COMMERCIAL & LITERARY
Between 80,000 and 89,999 words is a good range you should be aiming for. This is a 100% safe range for literary, romance, mystery, suspense, thriller and horror. Anything in this word count won't scare off any agent anywhere.
Now, speaking broadly, you can have as few as 71,000 words and as many as 109,000 words. That is the total range. When it dips below 80K, it might be perceived as too short—not giving the reader enough. It seems as though going over 100K is all right, but not by much. I suggest stopping at 109K because just the mental hurdle to jump concerning 110K is just another thing you don't want going against you. And, as agent Rachelle Gardner pointed out when discussing word count, over 110K is defined as "epic or saga." Chances are your cozy mystery or literary novel is not an epic. Rachelle also mentions that passing 100K in word count means it's a more expensive book to produce—hence agents' and editors' aversion to such lengths.
In short:
80,000 - 89,999: Totally cool
90,000 - 99,999: Generally safe
70,000 - 79,999: Might be too short; probably all right
100,000 - 109,999: Might be too long; probably all right
Below 70,000: Too short
110,000 or above Too long
Chick lit falls into this realm, but chick lit books tend to be a bit shorter and faster. 70-75K is not bad at all.
SCI-FI AND FANTASY
Science fiction and fantasy are the big exceptions because these categories tend to run long. It has to do with all the descriptions and world-building in the writing.
With these genres, I would say 100,000 - 115,000 is an excellent range. It's six-figures long, but not real long. The thing is: Writers tend to know that these categories run long so they make them run really long and hurt their chances. There's nothing wrong with keeping it short (say, 105K) in these areas. It shows that you can whittle your work down.
Outside of that, I would say 90K-100K is most likely all right, and 115-124K is probably all right, too. That said, try to keep it in the ideal range.
MIDDLE GRADE
Middle grade is from 20,000 - 45,000, depending on the subject matter and age range. When writing a longer book that is aimed at 12-year-olds (and could maybe be considered "tween"), using the term "upper middle grade" is advisable. With upper middle grade, you can aim for 32,000 - 40,000 words. These are books that resemble young adult in matter and storytelling, but still tend to stick to MG themes and avoid hot-button, YA-acceptable themes such as sex, drugs and rock & roll. You can stray a little over here but not much.
With a simpler middle grade idea (Football Hero or Jenny Jones and the Cupcake Mystery), aim lower. Shoot for 20,000 - 30,000 words.
YOUNG ADULT
Perhaps more than any other, YA is the one category where word count is very flexible.
For starters, 55,000 - 69,999 is a great range.
The word round the agent blogosphere is that these books tend to trending longer, saying that you can top in the 80Ks. However, this progression is still in motion and, personally, I'm not sure about this. I would say you're playing with fire the higher you go. When it gets into the 70s, you may be all right—but you have to have a reason for going that high. Again, higher word counts usually mean that the writer does not know how to edit themselves.
A good reason to have a longer YA novel that tops out at the high end of the scale is if it's science fiction or fantasy. Once again, these categories are expected to be a little longer because of the world-building.
Concerning the low end, below 55K could be all right but I wouldn't drop much below about 47K.
PICTURE BOOKS
The standard is text for 32 pages. That might mean one line per page, or more. 500-600 words is a good number to aim for. When it gets closer to 1,000, editors and agents may shy away.
MEMOIR
Memoir is the same as a novel and that means you're aiming for 80,000-89,999. However, keep in mind when we talked about how people don't know how to edit their work. This is specially true in memoir, I've found, because people tend to write everything about their life—because it all really happened.
Coming in a bit low (70-79K) is not a terrible thing, as it shows you know how to focus on the most interesting parts of your life and avoid a Bill-Clinton-esque tome-length book. At the same time, you may want to consider the high end of memoir at 99,999. Again, it's a mental thing seeing a six-figure length memoir.