How Long Should a Book Index Be?

Your index fills more pages than the publisher allowed. Or it looks suspiciously thin beside the book it serves. Before deleting entries or adding more, ask what is causing the length: useful coverage, repeated access, unnecessary structure or missing subjects? Each calls for a different repair.

A good index needs enough space to help its intended readers find the book’s important discussions. Page count and entry count can flag a problem. Neither can tell you whether those discussions are represented clearly, accurately and completely.

First, decide what you are counting

• Main headings provide starting points: heat pumps, insulation, tenants.
• Subheadings distinguish aspects within a subject: heat pumps—installation costs. More subheadings may clarify existing coverage without adding any new subject.
• Locators direct readers to treatments. The same page may legitimately appear under different subjects, so counting locator assignments is different from counting distinct pages.
• Typeset space is the room the index occupies after layout. Column width, wrapping, type size and indented or run-in subheadings affect it.

An estimate of ‘800 entries’ is ambiguous unless everyone agrees whether it includes subheadings and cross-references. Likewise, pages in a Word draft do not reliably predict pages in the printed book. Use a representative sample in the intended layout when space is tight.

Choose a working length for this book

The Society of Indexers’ guide to commissioning an index explains that there is no ideal length: book type and readership matter, and some scholarly or reference works need longer, more detailed indexes. It also recommends discussing space constraints with the indexer and, where necessary, the designer.

Start with the publisher’s requirements, the audience and the material actually in scope. Are substantive notes and captions included? Are subjects and names combined, or are there separate indexes? A word-count estimate should reflect that scope, rather than treating every page of bibliography or preliminary matter as equivalent to analytical text.

Subject complexity matters too. A methodological chapter may need many distinctions; a narrative passage may need fewer. Review coverage in proportion to what readers need to retrieve. Giving every chapter the same quota can leave the difficult chapter underindexed and the simpler one padded.

Shorten a large index without dropping subjects

Consider an imaginary book about home-heating policy. All examples and page numbers below are original illustrations. Its draft index repeats the same three subheadings under heat pumps, heat-pump systems and heat-pump technology:

• cold-weather performance, 90–94
• installation costs, 40–44
• maintenance, 110–113

Suppose the text uses those three names interchangeably for the same equipment and discussions. Keep the subheadings and locators under heat pumps, then replace the two repeated structures with:

• heat-pump systems. See heat pumps
• heat-pump technology. See heat pumps

Three main headings with three subheadings each become one developed heading and two redirects. All three search terms still work, and all treatments remain accessible. Repeating this repair across a large index can reduce bulk without removing subjects. Readers using a redirected term now take an extra step; retain direct locators where that convenience warrants the space.

Check meaning before merging. If technology covers the history of invention while systems covers installation design, those headings are doing different jobs. Matching page lists alone cannot establish that two subjects are synonyms.

Also inspect unnecessary hierarchy. A path such as heat pumps → performance → cold weather can become heat pumps → cold-weather performance when the middle level adds no useful choice. The useful distinction survives with less scaffolding. Follow the publisher’s permitted heading depth and keep subdivisions that help readers choose.

When a short index needs more

A short index can hide work from the reader in two ways. First, it may compress several discussions into an unhelpful locator string. In our imaginary book, retrofits, 20–24, 45–49, 70–74 gives no clue which passage addresses the reader’s question. If the source supports the distinctions, replace it with:

• retrofits—costs, 20–24
• retrofits—grants, 70–74
• retrofits—installation disruption, 45–49

The index grows, but its coverage has not changed. Readers can now choose the right treatment. Do not subdivide merely to meet a count; an integrated discussion may need no subdivision.

Second, the short index may omit important material. Suppose pages 140–149 analyze tenant protection during heating upgrades, but neither tenants nor a suitable related heading leads there. That is missing coverage. Adding tenant protection, 140–149, with suitable access under tenants, repairs it. Rearranging existing entries would leave the omission intact.

Check in both directions: follow entries into the book, then choose important discussions in the book and look for their index routes. UCL Press’s index submission checklist likewise treats coverage of main concepts and long locator strings as separate checks.

Decide what to repair

• ✓ Agree on scope, counting units and the publisher’s space limit.
• ✓ Consolidate genuine synonyms while preserving useful search routes.
• ✓ Collapse hierarchy that adds no meaningful choice.
• ✓ Subdivide overloaded headings where readers need different treatments.
• ✓ Recover important discussions that have no usable index access.
• ✓ Check the revised structure and locators, then assess the intended typeset layout.

If essential coverage still exceeds the allotted space, make that conflict visible to the production editor before cutting useful access. The right length is a considered fit between the book, its readers and its production constraints.