How many words should a middle grade novel be?

30K should be fine. Though I'll admit I tried this time to write shorter at 45K and have been asked by my editor to write more, so it really depends on the story you are telling and how you tell it.

I don't know what the wordcount is for chapter books but no you don't worry about illustrating anything. It's the same with picture books. You send them the words and then the publisher assigns an artist to the project.

Part of it depends on whether your book is more geared for lower or upper MG, and the genre. The ones whose word counts tend higher are upper MG, and those that are in the fantasy/ sci-fi genre can go higher still.

Now chapter books are a different matter, being a step between picture books and MG novels (books like Flat Stanley), and what I have read gives their range at 4,000-10,000 words. If your book is 30,000 words, it either isn't a chapter book or it is much too long for a chapter book, at least according to industry standards.

If you are not sure where you book falls in all of this, you need to find out before you submit. One thing I am pretty sure of is that there is an expectation in the publishing world that you be already familiar with the breakdown of different types of children's books and have some idea of where your book fits in it.

Adding onto all the very salient points jlmott made, when writing for kids each market has different writing expectations when it comes to word choice as well. Chapter books are going to have a very different vocabulary to a middle grade novel. Remember, kids are at different stages in their reading comprehension. It isn't just a question of wordcount that determines a chapter book vs a middle grade, it's also a matter of what the words themselves are.

Adding this link: http://literaticat.blogspot.com/2011/05/wordcount-dracula.html

I find it helpful as a reference point, though there are always exceptions, of course.

^^ Endearing beat me to it - that link is my go-to for word count.

I've been googling this a lot recently, and am finding 30,000-40,000 give or take 5,000 seems to be the right ballpark.

There have been some great replies already, but just to say, yes, 30-40k is fairly standard. If you're writing fantasy or science fiction, this will often go a lot higher. I know that Skellig by David Almond was a touch under 30k, and that felt perfect to me, so I aimed for 30k with my first MG book. Editing took it slightly higher, but if you can tell the story in 30k words, it's fine.

30,000 is perfect.