The plural of index is indexes or indices. Both forms are grammatically correct, but indexes is the safer choice in everyday, financial, publishing, and database writing, while indices is common in mathematics, science, statistics, and formal technical contexts.
Key Facts at a Glance
- Indexes and indices are both accepted plural forms of index.
- Indexes usually fits general English, stock-market writing, book references, and database documentation.
- Indices is traditionally associated with mathematics, physics, statistics, medicine, and other technical disciplines.
- The plural possessives are indexes’ and indices’, depending on which plural you choose.
- Indexs is incorrect because English adds -es after a word ending in -x.
- The pronunciation of indices is commonly “IN-duh-seez,” while indexes is “IN-dek-siz.”
What Is the Plural of Index?
The plural of index has two standard forms: indexes and indices. The form indexes developed as a regular English plural, whereas indices comes from the Latin plural traditionally used in scholarly language.
The choice depends primarily on meaning, audience, and field. A financial journalist would normally write “stock market indexes,” while a mathematician might write “summation indices.” A database administrator could use either form, but indexes is especially common in software documentation and product interfaces.
No grammatical rule makes indices more correct in every situation. Professional convention determines the preferred form.
Why Are There Two Plurals?
The two forms exist because English borrowed index from Latin and later treated it as an ordinary English noun. Latin nouns often entered English with their original plural endings, while everyday speakers created regular English forms with -es.
The same pattern appears in words such as:
- appendix: appendixes or appendices
- formula: formulas or formulae
- cactus: cactuses or cacti
- referendum: referendums or referenda
English allows both inherited and naturalized forms when usage establishes both. The technical field may preserve the older form, while general English favors the regular form.
Are Indexes and Indices Both Correct?
Yes. Indexes and indices are both correct plurals of index, although professional communities assign different preferences to them.
| Context | Preferred plural | Example | Reason |
|---|---|---|---|
| General writing | indexes | “The library has several indexes.” | Regular English plural |
| Investing | indexes | “Global stock indexes fell.” | Standard financial usage |
| Book publishing | indexes | “The two indexes occupy 12 pages.” | Common editorial usage |
| Databases | indexes | “The query uses three indexes.” | Common software terminology |
| Mathematics | indices | “The indices run from 1 to n.” | Established mathematical convention |
| Physics | indices | “Refractive indices vary by wavelength.” | Traditional scientific usage |
| Statistics | indices | “Price indices rose in June.” | Formal economic and statistical style |
| Medicine | indices | “Several health indices were compared.” | Common technical convention |
The table describes preference, not an absolute rule. A scientific publisher may prefer indexes if its house style standardizes regular plurals. Conversely, a general writer may use indices when discussing a specialized mathematical concept.
Which Form Is More Common?
Indexes is generally more common in broad modern English because it works across ordinary, commercial, and technical writing. Indices remains highly visible in disciplines that retain Latin-derived terminology.
Frequency varies by corpus and subject. A general newspaper, financial website, or software manual will usually favor indexes. A mathematics textbook, physics paper, or statistical report may favor indices. Search frequency alone cannot determine the best form for a particular sentence.
What Does Index Mean?
An index is a guide, indicator, list, or numerical measure that organizes information or represents a larger system. The plural changes spelling, but the underlying noun has several distinct meanings.
Index as a Reference List
A book index is an alphabetized list of subjects, names, places, or concepts, usually linked to page numbers. A scholarly index may include cross-references such as “climate change, see also carbon emissions.”
Example:
The textbook’s index lists “photosynthesis” on pages 84, 91, and 143.
A back-of-book index is not the same as a table of contents. A table of contents follows the book’s structure, while an index points readers to subjects wherever they occur.
Index as a Numerical Measure
A statistical or economic index summarizes change in a defined group of values. The Consumer Price Index, published by the U.S. Bureau of Labor Statistics, tracks changes in the prices paid by consumers for a basket of goods and services.
Example:
Consumer price indices increased during the quarter.
An index usually has a reference period or base value. If a base year is assigned a value of 100 and a later value is 125, the measured quantity is 25% higher than the base, assuming the index is constructed on that basis.
Index as a Database Structure
A database index is an additional data structure that helps a database engine locate rows without scanning an entire table. Common implementations include B-tree indexes, hash indexes, bitmap indexes, and full-text indexes.
Example:
The database indexes the
customer_idandcreated_atcolumns.
Database indexes can reduce read latency, but they require storage and can make INSERT, UPDATE, and DELETE operations slower because the index must also be maintained.
Index as a Mathematical Position
In mathematics, an index can identify a member of a sequence, tensor, matrix, or family of objects.
Example:
The indices (i) and (j) identify the row and column of a matrix.
In this context, indices is often preferred because it matches established mathematical vocabulary. The singular can also mean an exponent, as in “an index of 2,” although modern mathematics often uses exponent for that specific meaning.
When Should You Use Indexes?
Use indexes when writing for a general audience or when the subject is finance, publishing, databases, search, or ordinary information organization. Indexes is also the least distracting choice when readers come from several fields.
Examples include:
- “The exchange tracks four major stock indexes.”
- “The author added indexes for names and subjects.”
- “The application rebuilds its database indexes overnight.”
- “Search indexes store references to web documents.”
- “The report compares regional housing indexes.”
A regular English plural improves consistency when a document contains several meanings of index. It also avoids making a straightforward sentence appear unnecessarily specialized.
Does Finance Prefer Indexes?
Yes. Financial writing overwhelmingly uses indexes for groups of market indicators, including stock indexes, bond indexes, equity indexes, and volatility indexes.
Examples include:
- S&P 500 index
- Dow Jones Industrial Average
- Nasdaq Composite
- FTSE 100
- MSCI World Index
- Bloomberg U.S. Aggregate Bond Index
The plural spelling does not describe the weighting method. A market-cap-weighted index assigns greater influence to larger companies, while a price-weighted index assigns influence according to share price. Those are separate technical attributes.
| Financial term | Plural example | Weighting or role | Typical use |
|---|---|---|---|
| Stock index | stock indexes | Market performance measure | Equity reporting |
| Market-cap index | market-cap indexes | Company value determines weight | S&P 500-style construction |
| Price-weighted index | price-weighted indexes | Share price determines weight | Dow-style construction |
| Bond index | bond indexes | Tracks fixed-income securities | Fund benchmarking |
| Volatility index | volatility indexes | Measures implied market volatility | Risk analysis |
The Associated Press Stylebook commonly uses indexes in financial contexts. A financial firm may define a house style, but indices can sound unusual in a general investment article.
When Should You Use Indices?
Use indices when addressing a technical audience that expects the traditional scientific or mathematical form. The word is especially natural for mathematical subscripts, physical measurements, statistical measures, and formal scientific terminology.
Examples include:
- “The refractive indices of the materials were measured.”
- “The indices (i), (j), and (k) identify tensor dimensions.”
- “The researchers compared three biodiversity indices.”
- “The price indices use 2015 as the base year.”
- “The diagnostic indices showed moderate sensitivity.”
Indices signals disciplinary convention, but it does not automatically make prose more precise. If readers are not specialists, indexes may communicate more clearly without changing the meaning.
Which Plural Does Mathematics Use?
Mathematics commonly uses indices for variables that label terms, positions, dimensions, or components. A mathematical expression may contain a lower index, upper index, summation index, or multi-index.
Example:
For each index (i), the sequence contains a corresponding value (a_i).
The plural is also indices when discussing exponents or formal mathematical measures, although terminology differs between subfields. In elementary algebra, teachers often say “indices” in British English for powers, while American textbooks more often say “exponents.”
Mathematical style is not perfectly uniform. A programming text about array indexes may choose indexes, even when the underlying algorithm uses mathematical indices.
Which Plural Does Science Use?
Scientific writing often favors indices, particularly when the term forms part of a recognized technical expression. Examples include refractive indices in optics, ecological indices in environmental science, and metabolic indices in physiology.
The preference is strongest when the noun refers to formal measures or variables. It is weaker when index refers to a practical list, catalog, or software feature within a scientific organization.
What Is the Difference Between Indexes and Indices?
The main difference is register and field convention, not meaning. Indexes is the regular English plural, while indices is the traditional Latin-derived plural retained in many mathematical and scientific disciplines.
| Feature | Indexes | Indices |
|---|---|---|
| Grammatical status | Standard plural | Standard plural |
| Main register | General and professional English | Formal and technical English |
| Finance | Very common | Less common |
| Mathematics | Accepted, less traditional | Strongly established |
| Databases | Very common | Accepted but less usual |
| Pronunciation | IN-dek-siz | IN-duh-seez |
| Best default | Mixed or general audience | Specialist disciplinary audience |
Neither spelling changes the number of objects. “Two indexes” and “two indices” both mean two instances or collections of the thing called an index.
How Do You Choose the Right Form?
Choose the plural by identifying the meaning of index, the discipline, and the reader’s expectation. The following sequence works for editing, teaching, and technical documentation.
- Identify the sense. Decide whether index means a book list, market measure, database structure, mathematical label, or scientific measurement.
- Check the field convention. Finance and software generally favor indexes; mathematics and many sciences often favor indices.
- Match the audience. Use the form that readers will recognize without explanation.
- Follow the publication style guide. AP, Chicago, APA, a journal, or a company glossary may prescribe one form.
- Keep the choice consistent. Do not alternate between indexes and indices for the same sense in one document without a reason.
- Preserve quoted terminology. Do not silently alter an official product name, dataset title, or quotation.
A useful editorial rule is simple: use indexes by default, then change to indices when a specialist convention clearly supports it.
What If One Document Has Several Fields?
Use field-specific forms when the document genuinely discusses different technical meanings. For example, a financial report can refer to stock indexes and mathematical indices in a risk model.
If the distinction would confuse readers, define the choice in the style guide:
This report uses indexes as the general plural, including for statistical measures.
Consistency matters more than preserving every historical distinction. A document that uses one form predictably is easier to search, edit, translate, and index.
How Do You Pronounce Indices?
Indices is commonly pronounced IN-duh-seez in English. Indexes is pronounced IN-dek-siz.
Pronunciation can vary by country and discipline. In some technical settings, speakers may use a pronunciation closer to the Latin-derived spelling, but “IN-duh-seez” is the standard English pronunciation most readers will recognize.
| Word | Pronunciation guide | Example phrase |
|---|---|---|
| Index | IN-deks | “an index of topics” |
| Indexes | IN-dek-siz | “market indexes” |
| Indices | IN-duh-seez | “refractive indices” |
| Indexing | IN-dek-sing | “database indexing” |
Writers normally do not need to explain pronunciation unless the word will be read aloud, taught, or used in a presentation.
What Is the Plural Possessive of Index?
The plural possessive is indexes’ when the plural is indexes, and indices’ when the plural is indices. Both forms place the apostrophe after the plural word.
Examples:
- “The indexes’ closing values were published.”
- “The indices’ definitions appear in the appendix.”
- “The database indexes’ storage requirements increased.”
- “The statistical indices’ base years differ.”
The singular possessive is index’s:
“The index’s base year is 2015.”
Do not use indexs for either the plural or possessive. The spelling must retain the e before s.
Is Indexs Ever Correct?
No. Indexs is an incorrect spelling in standard English. The plural ending is -es, producing indexes, because index ends in the letter x and the added syllable makes the word pronounceable.
The same spelling rule produces:
- box, boxes
- tax, taxes
- fax, faxes
- suffix, suffixes
- index, indexes
The form indices is an alternative plural, not a correction of a supposed error in indexes. Writers should choose between indexes and indices, never between indexes and indexs.
What Are the Costs and Trade-Offs Behind Different Index Types?
The words indexes and indices do not themselves involve prices or implementation times. Costs belong to the activity represented by the noun, such as creating a book index, maintaining database indexes, or investing in index funds.
| Index activity | Typical cost or overhead | Typical timeframe | Main trade-off |
|---|---|---|---|
| Professional book index | $3-$6 per indexable page | 1-3 weeks | Higher quality, added publishing cost |
| 300-page book index | $900-$1,800 | 1-3 weeks | Better conceptual access than word search |
| Database index | No license cost in common SQL systems | Immediate creation, variable build time | Faster reads, slower writes |
| Database storage overhead | Typically 10%-30% for indexed structures | Ongoing | More disk use and maintenance |
| Index fund expense ratio | Often 0.03%-0.20% annually | 5-20+ year horizon | Diversification, market exposure |
| Statistical index construction | Project-specific labor and data cost | Weeks to months | Comparable measurement, modeling complexity |
These are typical planning ranges, not universal prices. A book’s subject complexity, a table’s row count, a fund’s asset class, and the chosen methodology can change the result substantially.
What Are the Common Indexing Mistakes?
The most common mistakes involve applying a convention from one field to another. A writer may call stock measures indices because the term sounds formal, or call mathematical labels indexes because regular English feels more familiar.
Practitioner rules help prevent avoidable errors:
- Do not treat indices as inherently more correct than indexes.
- Do not create a database index on every column without measuring query benefit.
- Do not build a book index by searching exact words only, because concepts may appear under synonyms.
- Do not mix plural forms randomly for the same meaning.
- Do not confuse an index with a table of contents, benchmark, catalog, or exponent.
- Do not assume a financial index is diversified merely because it contains many securities.
A book index requires human judgment about concepts and cross-references. A database index requires workload analysis, query plans, and write-performance testing. The shared word does not imply shared methods.
Which Form Should You Use in Specific Situations?
| Situation | Recommended form | Correct example | Editorial reason |
|---|---|---|---|
| General website | indexes | “Browse the site indexes.” | Clear regular plural |
| Stock-market article | indexes | “Asian indexes opened higher.” | Financial convention |
| Academic book back matter | indexes or house style | “The subject indexes begin on page 410.” | Publishing varies |
| SQL documentation | indexes | “Create indexes on filtered columns.” | Dominant software usage |
| Calculus textbook | indices | “The indices label each term.” | Mathematical convention |
| Optics paper | indices | “Refractive indices were calculated.” | Scientific terminology |
| Mixed business audience | indexes | “The report compares economic indexes.” | Broad readability |
| Formal statistical paper | indices | “The indices were normalized.” | Technical register |
The best choice is the one that minimizes reader friction while preserving the terminology of the field. Style guides can override the default, especially in academic journals, software projects, and multinational organizations.
FAQ
Is indices more correct than indexes?
No. Indices is not universally more correct than indexes. Indices is the traditional plural and remains preferred in many mathematical and scientific contexts, while indexes is the regular English plural used widely in general writing, finance, publishing, and databases.
What is the plural of a book index?
The plural of a book index is indexes or indices, but indexes is usually the more natural publishing term. For example, “The revised edition includes separate indexes for names and subjects.” A publisher’s style guide may choose indices, particularly for scholarly or international publications.
What is the plural of a database index?
The plural of a database index is most commonly indexes. SQL documentation, database administration guides, and software interfaces regularly use phrases such as “composite indexes,” “clustered indexes,” and “database indexes.” Indices is grammatically possible but less common in everyday programming documentation.
What is the plural of a stock-market index?
The standard financial plural is indexes. Write “stock indexes,” “market indexes,” or “equity indexes.” Indices may appear in formal financial research, but it is less typical in news reporting, investor education, and fund documentation.
Can I use both indexes and indices in one article?
Yes, when the words refer to different disciplinary conventions or meanings. Use indexes for a database or financial measure and indices for mathematical variables if those forms match the intended audiences. Otherwise, select one plural and apply it consistently throughout the article.
What is the plural of index in British English?
British English uses both indexes and indices. Usage depends on the field rather than geography alone. British mathematical writing often uses indices, while general, publishing, and computing contexts commonly use indexes. A British house style may specify one form for consistency.
The Bottom Line
The answer to “what’s the plural of index” is indexes or indices. Choose indexes as the general-purpose default, especially for finance, books, databases, and mixed audiences; choose indices when mathematical, scientific, statistical, or other specialist convention makes the Latin-derived form expected.
