Is Index Singular or Plural?

The short answer is that index is singular. Its standard plural form in general English is indexes, while in academic, mathematical, and technical writing, the plural indices is also correct and widely used. Which plural you choose depends on your context, audience, and the tone you want to set.

Quick Answer

• Singular: index
• Common plural (everyday English): indexes
• Formal/academic plural: indices
• Verb form: indexes (third person singular: indexes; past tense: indexed)

If you are writing an email to a colleague about a spreadsheet, use indexes. If you are writing a research paper in economics or mathematics, use indices. Both are correct, but they carry different levels of formality.

Understanding the Two Plurals

The word index comes from Latin, which is why it has two plural forms: the English-style plural indexes and the Latin-style plural indices. This dual plural is common for Latin-derived words (like appendix → appendices or appendixes, and matrix → matrices or matrixes).

When to Use “Indexes”

Indexes is the preferred plural in general, non-technical writing. It sounds natural in conversation, business emails, and most everyday contexts. If you are unsure which plural to choose, indexes is almost always a safe option.

• Formal level: Neutral to informal
• Best for: Emails, conversations, general articles, business reports
• Example: “The book has two separate indexes at the back.”

When to Use “Indices”

Indices is the preferred plural in academic, scientific, and technical writing. You will see it in mathematics (e.g., indices of a matrix), economics (e.g., price indices), and formal research papers. Using indices signals that you are writing for a specialized audience.

• Formal level: Formal to very formal
• Best for: Research papers, textbooks, technical documentation, academic journals
• Example: “The economic indices show a steady recovery.”

Comparison Table: Indexes vs. Indices

Natural Examples

Here are examples showing how index, indexes, and indices appear in real writing and conversation.

Singular: Index

• “Can you add this term to the index?”
• “The index at the back of the book is incomplete.”
• “This index measures consumer confidence.”

Plural: Indexes (everyday use)

• “I created three separate indexes for the report.”
• “The database indexes are updated every night.”
• “Please review the indexes before we publish.”

Plural: Indices (formal/academic use)

• “The economic indices suggest inflation is slowing.”
• “In mathematics, we use subscripts to denote indices.”
• “The research paper includes several indices of performance.”

Common Mistakes

Even advanced learners sometimes make errors with index. Here are the most frequent mistakes and how to avoid them.

Mistake 1: Using “indices” in casual conversation

Incorrect: “I need to check the indices in my phone’s contact list.”
Correct: “I need to check the indexes in my phone’s contact list.”
Why: In everyday contexts, indices sounds overly formal and unnatural.

Mistake 2: Using “indexes” in a formal academic paper

Incorrect: “The study uses several economic indexes to measure growth.”
Correct: “The study uses several economic indices to measure growth.”
Why: In academic economics, indices is the standard term.

Mistake 3: Treating “index” as plural

Incorrect: “These index are outdated.”
Correct: “These indexes are outdated.” or “These indices are outdated.”
Why: Index is singular; it needs a plural form.

Mistake 4: Mixing singular and plural in the same sentence

Incorrect: “The index for each chapter are listed separately.”
Correct: “The indexes for each chapter are listed separately.”
Why: The subject and verb must agree in number.

Better Alternatives and When to Use Them

Sometimes you may want to avoid the indexes vs. indices decision entirely. Here are alternatives that work in specific contexts.

For book references

• “Table of contents” (for chapter-level navigation)
• “Glossary” (for definitions of terms)
• “Concordance” (for a detailed word index in literary works)

For data and measurements

• “Metrics” (for performance measurements)
• “Indicators” (for economic or social measures)
• “Benchmarks” (for comparison standards)

For database or computer contexts

• “Lookup tables”
• “Directories”
• “Catalogs”

Use these alternatives when you want to be more specific or when the word index might cause confusion.

Formal vs. Informal Tone in Writing

Choosing between indexes and indices is also a matter of tone. Here is how to match your choice to your writing situation.

Email and conversation (informal to neutral)

Use indexes. It sounds natural and unpretentious.

• “I updated the indexes in the shared folder.”
• “Can you double-check the indexes before the meeting?”

Academic papers and reports (formal)

Use indices. It matches the expected style in most disciplines.

• “The volatility indices were calculated using daily returns.”
• “Table 3 presents the correlation indices for all variables.”

Technical documentation (neutral to formal)

Both are acceptable, but indices is more common in mathematics and computer science.

• “The array indices start at zero.” (formal)
• “The array indexes start at zero.” (acceptable but less common)

Mini Practice: Check Your Understanding

Choose the correct form of index for each sentence. Answers are below.

1. The book has three separate ______ at the back. (indexes / indices)
2. The economic ______ published by the government show a positive trend. (indexes / indices)
3. Please update the ______ in the spreadsheet before the deadline. (index / indexes)
4. In mathematics, we use subscripts to represent ______. (indexes / indices)

Answers

1. indexes (general book reference)
2. indices (formal economic context)
3. indexes (everyday business context)
4. indices (mathematical/technical context)

Frequently Asked Questions

1. Is “index” ever used as a verb?

Yes. Index can be a verb meaning to create an index or to adjust for inflation. For example: “The software indexes all files automatically.” The verb form always uses indexes (third person singular) and indexed (past tense).

2. Can I use “indices” in a business email?

It depends on your audience. If you are writing to colleagues in finance or economics, indices is appropriate. For general business communication, indexes is safer and sounds more natural.

3. Which plural is more common in American English?

In American English, indexes is more common in everyday writing, while indices is reserved for academic and technical contexts. British English follows a similar pattern, though indices may appear slightly more often in formal British writing.

4. What about the word “index” in search engines?

In the context of search engines (like Google), the plural is almost always indexes. For example: “The search engine indexes billions of web pages.” Using indices in this context would sound odd and overly technical.

Final Note

Remember that both indexes and indices are correct plurals of index. Your choice should be guided by your audience and the formality of your writing. When in doubt, indexes is the safer, more natural choice for most situations. For academic or technical work, indices shows that you understand the conventions of your field.

For more help with similar questions, explore our guides on Singular or Plural Checks and Confusing Plurals. If you have further questions, visit our FAQ page or contact us.