Plural of Index
“Index” has two accepted plurals. In ordinary list/catalogue and many business uses, “indexes” is common; “indices” is widely used in mathematical, statistical, and technical contexts. The choice is therefore partly sense- and register-dependent rather than a simple right/wrong contrast.
Both “indexes” and “indices” are accepted plurals of “index.” “Indexes” is especially natural for alphabetical or database lists; “indices” is common in mathematical, statistical, and other technical contexts. Follow the sense and the convention of your field.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| book/database lists | indexes | A common and direct plural for list senses. |
| technical mathematical/statistical uses | indices | Common in specialized contexts. |
| general use with no fixed convention | indexes or indices | Both are accepted; follow context and consistency. |
Quick tests
Plural-choice test
Are you writing in a technical field that conventionally uses “indices”?
If yes: Use “indices” if that field convention applies.
If no: For ordinary list/catalogue senses, “indexes” is a clear default; both forms remain valid overall.
Caveat: This is a sense/register preference, not a universal correctness rule.
Examples
The book contains separate author and subject indexes.
“Indexes” fits the list sense naturally.
The model compares several price indices.
“Indices” fits a technical/statistical context.
The database maintains multiple indexes for faster lookup.
“Indexes” is standard in many computing/database contexts.
Further guidance
Core difference
Use “indexes” freely for lists and catalogues; use “indices” where technical or field convention favors it. Do not treat either as universally mandatory.
Fast Test
If the noun names an alphabetical/database list, “indexes” is a safe default. In technical mathematics/statistics or specialized writing, check field convention for “indices.”
Usage Boundary
Because both forms are current, consistency and sense matter more than inventing a global rule that one is always correct.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- INDEX | English meaning - Cambridge Dictionary (opens in a new tab)Cambridge University Press · Cambridge gives indices and indexes as plurals and its Business English treatment distinguishes common sense preferences, supporting a sense-aware variant guide.