[Rules of the Road is an occasional segment of the Writer's RoadTrip. I'll discuss a selected writing "rule" and talk about its underlying rationale. This knowledge in hand, writers can be more confident when breaking the rules.]
Faced with the idea that there is some set standard, what writer hasn't asked at some point, "How long is a chapter?" And while there are many opinions and no simple answer, there are a few things to consider when thinking about chapter length and, just as importantly, chapter length consistency.
But first, what should a chapter contain? In essence, a chapter really has to encompass an entire scene, in some cases more than one. Unfortunately, the term "scene" can be broadly defined, which makes the task more difficult. It can be as simple as something the MC does ("Tuesday morning I got up and went to Fluffy's funeral."), or be as complex and drawn out as the Battle of the Midway.
I like to think of a scene in terms of a package with three pieces:
1) it begins by describing some new problem,
2) raises the stakes for your MC, and
3) concludes at some critical decision or turning point.
The next scene will do the same. If your chapter doesn't accomplish these three things at least once, you've cut off your scene prematurely. If your chapter contains more than one scene, consider whether they are related and contribute in a way that they build upon each other. Readers have become trained to expect this.
OK, but how long should a chapter be? Well, obviously that first depends on your target audience. Shorter chapters for younger readers, longer for older readers with longer attention spans. Second, sophistication and treatment of material. Early readers/chapter books need to be straightforward; YA and adult novels... well, you get the picture.
Often ignored is the impact of reader tolerance. Your nine-year-old isn't going to have the patience to slog through a 5000-word chapter. And your adult novel isn't going to seem meaty enough if the chapters top out at 500 words.
I try to adhere to the following guidelines:
200-500 words for early chapter books
500-1500 for middle grade
1500-2500 for tween
1500-3500 for YA
2000-4500 for adult
These are just guidelines I set for myself for the types of stories I write. Your numbers may differ. Of course, other factors will impact these numbers for you, primarily your style. But not everyone is Cormac McCarthy. Not everyone can get away with not having chapters at all.
Now, on to chapter length consistency.
I've seen very strong opinions on this, most on the side of keeping chapter lengths within a particular work within a narrow range. "Avoid varying chapters by much more than +/-10%." In other words, in a 75K-word novel with an average chapter length of 3000 words, chapters should range from 2700-3300 words. I think this is a bit tight, though I do agree in priniciple with its rationale: Readers develop a sense of timing and rhythm after the first few chapters, and set their expectations for later chapters. If a chapter is much shorter than expected, the reader feels cheated. If much longer, he begins to get antsy, lose patience and attention. And you don't want your reader to feel any of those things.
But does that mean every chapter needs to conform to some narrow range? Does it mean I need to expand my 1200-word scene so it falls within my 3000-word average +/- 300 words? Of course not. The scene must work as best written. Just keep in mind what effect it might have if uncharacteristically long or short. If short, make it rewarding enough for the reader, which means accomplishing those three things I mentioned above. If much longer, make sure it captures and holds onto your reader's attention.
One last thought. Layered over all of this is the structure of the entire work. In general, longer chapters slow pace and are more appropriate in the early and middle sections of the book; shorter chapters pick up the pace and are more appropriate for building up to the climax.
So, now you know. Which means you won't have to be a rule robot. If your work demands an unconventional chapter structure, great. The Writing Police have no juristiction here. Just as long as you understand the impact and can deal with them effectively.