Index Plural Form: Rule, Examples, and Mistakes

The plural form of index is not as simple as adding an -s. In fact, index has two accepted plurals: indexes and indices. The choice depends on context, tone, and field of use. Indexes is the standard plural for everyday English, especially in business, computing, and general writing. Indices is the traditional Latin plural, used primarily in formal, academic, or technical contexts such as mathematics, finance, and statistics. This guide explains the rule, gives clear examples, highlights common mistakes, and helps you choose the right form for your writing.

Quick Answer: Which Plural Should You Use?

Use indexes for most business emails, reports, website navigation, and general conversation. Use indices when writing about financial market indicators, statistical measurements, or in formal academic papers. If you are unsure, indexes is almost always safe and natural in modern English.

Understanding the Two Plurals of Index

The word index comes from Latin, where its plural was indices. Over time, English speakers began applying the regular -es plural rule, creating indexes. Today, both forms are correct, but they carry different connotations.

Indexes: The Everyday Plural

Indexes follows the standard English plural rule for words ending in -x (add -es). It is the preferred form in:

• Business writing and emails
• Computer science and software documentation
• Website navigation (e.g., “site indexes”)
• Informal and semi-formal conversation
• General non-specialist publications

Indices: The Formal and Technical Plural

Indices retains the original Latin inflection. It is the standard choice in:

• Financial reports (e.g., “stock market indices”)
• Academic research papers
• Mathematics and statistics
• Scientific publications
• Formal legal documents

Comparison Table: Indexes vs. Indices

Natural Examples in Context

Using “Indexes” in Business and Everyday Writing

• “Please update the project indexes before the quarterly review.” (Business email, neutral tone)
• “The database has three separate indexes for faster searches.” (Technical documentation, standard usage)
• “We created new indexes for the customer feedback forms.” (Office conversation, natural)
• “Check the website indexes to ensure all pages are listed.” (Web development, common)

Using “Indices” in Formal and Technical Writing

• “The major stock market indices showed a decline this quarter.” (Financial report, formal)
• “The study used multiple economic indices to measure growth.” (Academic paper, technical)
• “Calculate the refractive indices for each material.” (Scientific context, precise)
• “The consumer price indices were adjusted for inflation.” (Statistical analysis, formal)

Mixed Context: When Either Form Works

• “The report includes several performance indexes / indices.” (Both acceptable; indexes sounds more direct, indices more formal)
• “We compared the indexes from last year.” (Conversational, clear)
• “We compared the indices from last year.” (Slightly more academic tone)

Common Mistakes with the Plural of Index

Mistake 1: Using “Indices” in Casual Business Emails

Incorrect: “Please send me the sales indices for this month.”
Correct: “Please send me the sales indexes for this month.”

Why: In a routine business request, indexes sounds natural and professional. Indices can feel stiff or overly academic for everyday communication.

Mistake 2: Using “Indexes” in Formal Financial Reports

Incorrect: “The market indexes fell by 2%.”
Correct: “The market indices fell by 2%.”

Why: In finance and economics, indices is the established convention. Using indexes here may mark you as unfamiliar with the field.

Mistake 3: Adding an Apostrophe

Incorrect: “The index’s are updated weekly.”
Correct: “The indexes are updated weekly.”

Why: Apostrophes indicate possession, not plurals. This is a common punctuation error.

Mistake 4: Using “Indice” as a Singular

Incorrect: “This indice is important.”
Correct: “This index is important.”

Why: The singular form is always index. Indice is not a standard English word.

Better Alternatives and When to Use Them

Sometimes you can avoid the plural choice altogether by rephrasing. Consider these alternatives:

• “List” or “table of contents” – Use when referring to a book or document’s index.
• “Database keys” – Use in computing contexts instead of “indexes.”
• “Market benchmarks” – Use in finance instead of “indices” for clarity.
• “Statistical measures” – Use in research instead of “indices.”

When to use the original plural: Stick with indexes or indices when the term is standard in your field or when you need precise technical language. Rephrasing is helpful if you are writing for a mixed audience and want to avoid confusion.

Formal vs. Informal Tone: Choosing the Right Plural

Your choice of plural can subtly affect the tone of your writing:

• Informal conversation: “We need to update the indexes.” (Direct, friendly)
• Business email to a colleague: “Could you check the indexes for errors?” (Professional, approachable)
• Business email to a client: “Our analysis includes several key indexes.” (Neutral, clear)
• Formal report: “The economic indices indicate a recovery.” (Authoritative, precise)
• Academic paper: “The refractive indices were measured.” (Technical, standard)

In email, using indexes keeps the tone warm and efficient. Using indices in an email to a financial client shows expertise. Match the form to your reader’s expectations.

Mini Practice: Test Your Understanding

Choose the correct plural form for each sentence. Answers are below.

1. The company’s performance (indexes / indices) improved this quarter. (General business context)
2. The researcher calculated several economic (indexes / indices) for the study. (Academic context)
3. Please update the database (indexes / indices) after adding new records. (Computing context)
4. The stock market (indexes / indices) were mixed today. (Financial context)

Answers:

1. indexes – General business writing favors the regular plural.
2. indices – Academic research typically uses the Latin plural.
3. indexes – Computing and database documentation use indexes.
4. indices – Financial markets conventionally use indices.

Frequently Asked Questions (FAQ)

1. Is “indexes” ever wrong?

No. Indexes is a standard, correct plural in modern English. It is the preferred form in most non-specialist writing. Only avoid it if your field or publication style guide specifically requires indices.

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

Yes, but consider your audience. If you are writing to a financial professional or in a formal context, indices is appropriate. For general business communication, indexes is usually more natural.

3. What is the plural of “index” in a book?

For a book’s back-of-the-book list, both indexes and indices are used, but indexes is far more common in publishing. Most style guides for general publishing recommend indexes.

4. How do I pronounce “indices”?

Indices is pronounced /ˈɪn.dɪ.siːz/ (IN-di-seez), with the emphasis on the first syllable. The -ces sounds like “sees.” Indexes is pronounced /ˈɪn.dɛk.sɪz/ (IN-dek-siz), with a hard ks sound.

Final Recommendation

For most writing at work, in emails, and in everyday conversation, choose indexes. It is clear, correct, and avoids sounding pretentious. Reserve indices for formal financial, mathematical, or academic writing where the term is standard. When in doubt, indexes is the safer and more modern choice. If you need more guidance on plural forms, explore our Plural Spelling Rules section or visit our FAQ page for common questions. For other tricky plurals, check our Confusing Plurals category. We also have resources on Common Plural Forms and Singular or Plural Checks to support your learning.