How do you know if your manuscript fits into the middle-grade category? Although no rule is carved in stone, these general guidelines should help:
1) The main character is 8-13 years old. Most children like to read about characters their own age or older, so it’s best to shoot for the upper range here.
2) The manuscript is approximately 20,000-60,000 words. Again, there are loads of examples with higher word counts and lesser word counts. But this a good general rule of thumb. Or Foot. As long as it doesn’t stink. [For some bonus thoughts on this topic, check out the following post by MUF's Hillary Homzie: "How Long Is Middle Grade Fiction?"]
3) The story doesn’t contain issues of an overtly sexual or violent nature. So save that kissing for your young adult manuscript. And keep the zombie killing to a minimum. For the zombies’ sake.