Number chapters in Arabic numerals. The word "Chapter" before the number is optional. Parts, if you have them, take Roman numerals.
That is the convention as stated by New Hart's Rules, and it is one of the few things in this territory with an actual source.
The Convention
New Hart's Rules, the Oxford style guide, sets it out:
"Most works in prose are divided into chapters, which usually have a number (customarily in Arabic numerals) and often, especially in non-fiction, a title. The use of the word Chapter before the number is optional."
Note the aside about titles: chapter titles are more expected in non-fiction than in fiction. A novel with numbers alone is entirely normal.
For parts, the same guide gives the split:
"It is customary to use upper-case Roman numerals for part numbers and Arabic numerals for chapter numbers."
And a consistency rule that catches people out:
"The word Chapter may, but need not, appear before the number and title of each chapter, though if it is used in the list of contents it should also appear at the head of each chapter in the text."
So the contents page and the chapter openings must agree. If your contents lists "Chapter One", the chapter itself cannot open with a bare "1".
Numerals or Words
No authority states a preference. The Chicago Manual of Style's free Q&A has no entry on chapter numbering at all, and its body text is paywalled. New Hart's Rules says "customarily in Arabic numerals" without addressing whether to spell them out.
Both "1, 2, 3" and "Chapter One, Chapter Two" appear in published fiction and both are correct. Pick one and hold it.
There is one punctuation rule, from the Chicago editors' blog: "Chapter numbers are rarely followed by a period unless a chapter title follows." So "1" and "Chapter One" take no full stop, but "1. The Drowned Meadow" does.
Titles, Numbers, or Both
The Chicago editors settled this by sampling rather than ruling, checking fourteen New York Times bestselling novels:
"4 novels used chapter (or part) numbers but no titles, 4 novels used chapter titles but no numbers, 6 novels combined numbers and titles for parts and chapters."
An even three-way split, with their conclusion: "there is a choice, even within traditional publishing".
Two practical notes from the same source. Titles help a reader relocate a passage, "especially where chapters alternate between their points of view". And two warnings: "Think twice about including essential information in the chapter title" and "Avoid spoilers." A chapter titled "Marianne's Funeral" has told the reader something the chapter was presumably going to.
One more: "Don't end chapter titles with a period."
Fuller treatment in chapter titles vs chapter numbers.
Where the Number Sits on the Page
New Hart's Rules covers the page treatment, which is a separate decision from the numbering itself:
"New chapters are usually allowed to begin on either a verso or a recto (unlike new parts); exceptionally, when chapters are short or economical setting is required, they may run on, start on the same page as the end of the preceding chapter, after a specified number of lines' space. This is more common in fiction than in non-fiction."
"The first page of a new chapter lacks a running head, and the folio is either omitted or appears at the foot of the page (as a drop folio)."
Both details are easy to get wrong by hand and are what make a self-typeset book look self-typeset: a running header on a chapter opening page, or a page number sitting at the top of it.
What Deckle Does
Chapter numbering is automatic and configurable, so splitting chapter twelve does not mean renumbering everything after it.
Five number formats are available: Arabic (the default), spelled out, Roman, uppercase, or hidden entirely. The "Chapter" label is a separate toggle, matching the convention that the word is optional.
Because the generated table of contents is built from the same settings, the contents page and the chapter openings cannot drift apart, which is the consistency rule New Hart's Rules states.
Two related settings handle the page treatment above. Running headers are suppressed on chapter opening pages automatically. And the recto chapter start option controls whether chapters open on a right-hand page or run on, which is the fiction-versus-non-fiction difference New Hart's Rules describes. See why chapters start on a right-hand page.
For a prologue or epilogue you will want numbering hidden for that document, so the heading reads "Prologue" rather than continuing the chapter sequence.
Common Mistakes
- Contents and chapter openings disagreeing. If the contents says "Chapter One", the chapter must too.
- Roman numerals for chapters. Convention gives Roman to parts and Arabic to chapters.
- A full stop after a bare number. Only used when a title follows.
- A full stop at the end of a chapter title. Explicitly advised against.
- Numbering by hand. One split chapter means renumbering the rest.
- A running header on a chapter opening page. It should be absent.
FAQ
Q: Arabic or Roman numerals for chapters? A: Arabic. Roman numerals are conventionally for parts.
Q: "1" or "Chapter One"? A: Both are correct and no authority states a preference. Be consistent.
Q: Do I need the word "Chapter"? A: Optional, but if it appears in the contents it must appear at the chapter head too.
Q: Should chapter numbers have a full stop? A: Only when a title follows, as in "1. The Drowned Meadow".
Q: How do I number a prologue? A: Do not. Title it and turn numbering off for that document.