Judging from Facebook writers’ groups, chapter lengths are one of the main anxieties of new writers. Hardly a week goes by without someone obsessing about whether their chapters are too short, or too long, and craving for a scrap of certainty that, frankly doesn’t exist. I could say that I have seen chapters of two words and others of ten thousand words, or that the average length appears to be 3000-4500 words, but neither of those answers is very useful. Like most aspects of writing, chapter length is highly circumstantial.
One school of thought is that you should take pity on your readers, and keep your chapters short for the sake of those who want to finish a chapter before going to bed. Most people would also agree that chapters in a children or young adult book should be shorter than those in a book for an older audience. But these answers aren’t especially useful either. How short is shorter, or long is longer?
To get an actual length, you might start with with your structure. For example, if you are using a five act structure borrowed from Shakespearean plays, then you could plan on five scenes per act, and plan each scene as a chapter. If you plan on a 100,000 word novel, that means your chapters should average out to 4,000 words. The trouble is that Shakespeare himself rarely wrote a perfectly symmetrical play, and frequently had Acts with three or seven chapters in them. Nor are other standard structures any more useful.
Lacking a firm answer, I prefer to plan my chapter lengths by the rhythm they create. Chapters are a natural break in the narrative, and a fresh start. Just as short sentences have a different effect than longer sentences, so a short chapter has a different effect than a longer one. On the one hand, consistently short chapters are likely to create a faster pace, perhaps with more changes in point of view. If you want to go into more depth with shorter chapters, you may need more cliff-hanger endings — and, even if you don’t, the narrative will sometimes spill over into the next chapter, and is likely to break the rhythm established by earlier chapters (which may or may not be something you want to do). On the other hand, long chapters are apt to be slower, and perhaps more philosophical. Your chapters are more likely to come to definite ends.
However, who says that chapters have to be a consistent length? A single sentence chapter can be used for a number of different purposes. In her Falco series, for example, mystery writer Lindsey Davis ends on chapter with a thrown knife, and deliberately breaks the tension in the next, single-sentence chapter in which her narrator simply says that he caught it. Suspense is not the main point of the narrative at that point, so Davis refuses to milk it. More recently, in The Cruel Prince, Holly Black glosses over a lapse of a decade with the simple sentence, “In Faerie, there are no fish sticks, no ketchup, no television,” implying all that a young girl might miss growing up among elves in just eleven words. Each is brilliant in its own way, even if I suspect both writers delight in showing off their writing skills.
Similarly, a chapter longer than those around it can also be useful. Consider, for example, The Council of Elrond in The Fellowship of the Ring. It follows the tense retreat of Aragorn and the four hobbits from Weathertop, racing against time so that Frodo can get the healing he requires. In fact, ever since Bree, the hobbits have been pursued by the Black Riders, with next to no relief of tension. Once they are under Elrond’s protection, the tension is broken, and the characters and the readers alike are overdue for a rest. So Tolkien gives them one, full of welcome reunions and pages of history and debate which, if sometimes ominous, seem remote when heard in such a refuge.
Tolkien also makes effective use of a long chapter in The Battle of Pelennor Fields. The chapter describes the siege of Minas Tirith by Sauron’s army, and the mood gradually becomes grimmer and more hopeless as the city fights on, waiting for the forces of Rohan to relieve it. Just as the tension has been cranked up to an unbearable pitch, a cock crows, and the long chapter ends with, “Rohan had come at last.”
As these examples show, how long your chapters should be is not a trivial question. The problem is that it has no easy answer. The only meaningful answer is: that depends on what you are trying to do.