What Is the Plural of Index?

The word index has two correct plural forms: indexes and indices. Both are widely used, but they are not always interchangeable. The choice depends on context, audience, and the specific field you are writing in. Indexes is the standard plural in general English and computing, while indices is the preferred form in mathematics, finance, and academic writing. This guide will help you choose the right form for your business writing, emails, and everyday communication.

Quick Answer: Indexes vs. Indices

Here is the simplest way to decide which plural to use:

• Indexes – Use for general English, lists, tables of contents, and database terms. Example: “The book has two indexes: one for names and one for topics.”
• Indices – Use for mathematical, financial, or scientific contexts. Example: “The stock market indices fell sharply yesterday.”

If you are unsure, indexes is almost always safe in everyday business writing. In formal reports or technical documents, check your industry style guide.

Detailed Explanation of Both Plurals

Indexes (General and Computing)

Indexes follows the regular English rule of adding -es to words ending in -x. This form is common in:

• Books and publishing: “The publisher added three indexes to the encyclopedia.”
• Databases and software: “The database administrator rebuilt the indexes to improve query speed.”
• Everyday conversation: “I checked the indexes in the back of the manual.”

In business emails, using indexes sounds natural and avoids confusion. For example: “Please update the price indexes in the spreadsheet before the meeting.”

Indices (Formal, Academic, and Technical)

Indices comes from the Latin plural of index. It is the standard form in:

• Finance and economics: “The consumer price indices rose by 0.3% last quarter.”
• Mathematics and science: “The refractive indices of the two materials are different.”
• Academic writing: “Several indices of economic growth were examined in the study.”

Using indices in a formal report signals precision and familiarity with technical terminology. However, using it in a casual email to a colleague might sound overly academic.

Comparison Table: Indexes vs. Indices

Natural Examples in Context

Seeing both forms in real sentences helps you choose the right one. Here are examples from different situations:

• Business email (informal): “Could you check the price indexes for last month? I think there might be an error in row 12.”
• Business report (formal): “The economic indices for the third quarter indicate a slowdown in manufacturing.”
• Technical documentation: “Rebuilding the database indexes can improve performance by up to 40%.”
• Academic paper: “We calculated the diversity indices for each sample site.”
• Everyday conversation: “I looked at the indexes in the back of the cookbook to find the recipe for lasagna.”

Common Mistakes with the Plural of Index

Even experienced writers sometimes mix up these forms. Here are the most frequent errors and how to avoid them:

• Mistake 1: Using “indices” in a general context. Example: “The book has three indices.” Correction: “The book has three indexes.” Unless you are writing about a mathematical or financial book, indexes is better.
• Mistake 2: Using “indexes” in a formal financial report. Example: “The stock market indexes were volatile.” Correction: “The stock market indices were volatile.” In finance, indices is the standard term.
• Mistake 3: Writing “index’s” as a plural. Example: “The index’s are listed on page 5.” Correction: “The indexes are listed on page 5.” The apostrophe shows possession, not plurality.
• Mistake 4: Using “indices” as a singular. Example: “This indices is incorrect.” Correction: “This index is incorrect.” Indices is always plural.

Better Alternatives and When to Use Them

Sometimes you can avoid the confusion altogether by using a different word. Here are some alternatives:

• List – Use when referring to a simple collection of items. Example: “Please check the list of prices.” This works in any context.
• Table of contents – Use for books or long documents. Example: “The table of contents is on page iii.”
• Catalog – Use for a systematic list of items. Example: “The product catalog is updated quarterly.”
• Indicator – Use in financial or scientific contexts. Example: “The economic indicators suggest growth.”

When you need to be precise, stick with indexes or indices. But if you are writing for a general audience and want to avoid confusion, a simpler word like list or catalog can be a good choice.

Mini Practice: Test Your Knowledge

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

1. The database administrator rebuilt the ______ to speed up searches. (indexes / indices)
2. The financial analyst studied several market ______ before making a recommendation. (indexes / indices)
3. Please update the price ______ in the shared folder. (indexes / indices)
4. The refractive ______ of the two liquids were measured in the lab. (indexes / indices)

Answers:

1. indexes – This is a computing context, so the regular plural is standard.
2. indices – This is a financial context, so the Latin plural is preferred.
3. indexes – This is a general business context, so the regular plural is natural.
4. indices – This is a scientific context, so the Latin plural is correct.

Frequently Asked Questions

1. Is “indexes” ever wrong?

No, indexes is never wrong in general English. It is the standard plural for most contexts. The only time it might be considered incorrect is in very formal academic or technical writing where indices is the expected form.

2. Can I use “indices” in an email to my boss?

It depends on your workplace. If your boss works in finance, economics, or a technical field, indices is appropriate. In a general office setting, indexes is safer and sounds more natural.

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

In computing and search engine contexts, the plural is almost always indexes. For example: “Google’s indexes are updated constantly.” Using indices here would sound unusual.

4. Are there any other words like “index” with two plurals?

Yes, several words from Latin have both regular and irregular plurals. Common examples include appendix (appendixes / appendices), matrix (matrixes / matrices), and vertex (vertexes / vertices). The same rule applies: use the regular form for general contexts and the Latin form for technical or academic writing.

Final Tip for Business Writers

When you are writing a business email, report, or document, think about your audience. If you are communicating with colleagues in a general office setting, use indexes. If you are writing for a financial, scientific, or academic audience, use indices. When in doubt, check your company’s style guide or look at how similar documents are written in your field. Consistency is more important than choosing one form over the other.

For more guidance on plural forms in business English, explore our resources on Common Plural Forms and Confusing Plurals. If you have questions about this topic, please visit our FAQ page or contact us directly.