Indexes or Indices: Which Is Correct?

When you are unsure whether to write “indexes” or “indices,” you are dealing with two accepted plural forms of the word “index.” Both are correct, but they are used differently depending on context and style. In general English, indexes is common when talking about lists, especially book indexes, while indices is frequently used in mathematics, statistics, science, and economics.

The choice can also depend on the meaning of index. For example, a book may contain several indexes, while a mathematician may work with numerical indices. Understanding the context makes the choice much easier. This guide explains the difference between indexes and indices, when to use each form, and how to avoid common mistakes.

Quick Answer

Indexes and indices are both correct plural forms of index. Use indexes commonly for book or database lists, and use indices especially in mathematics, science, statistics, and economics. However, indices can also be used more broadly, depending on the context and style.

Why Do People Confuse It?

The confusion comes from the fact that index has two standard plural forms:

• Indexes — the regular English plural.
• Indices — a traditional plural borrowed from Latin.

Both forms are grammatically correct.

The main difference is often context rather than correctness. Certain fields traditionally prefer indices, while everyday English frequently uses indexes.

For example:

But:

First Word: Indexes

Indexes is the plural form of index that follows the normal English pattern of adding -es.

It is especially common when index means an alphabetical or organized list that helps you find information.

Examples:

• The library has detailed indexes of its journals.
• The book contains separate indexes for names and subjects.
• Search engines create indexes of web pages.
• The database contains several indexes.

Indexes can also be used in technical contexts. It is not incorrect simply because the subject is mathematical or scientific.

Second Word: Indices

Indices is another plural form of index, derived from Latin.

It is particularly common in technical and academic fields such as:

• Mathematics
• Statistics
• Economics
• Physics
• Engineering
• Finance
• Science

For example:

• The economic indices changed during the year.
• Different mathematical indices are used in the calculation.
• Researchers compared several health indices.

In some specialized contexts, indices may sound more natural because it is the traditional terminology of that field.

Indexes vs Indices: Comparison Table

Feature	Indexes	Indices
Singular	Index	Index
Plural	Indexes	Indices
Origin	English plural formation	Latin-derived plural
Common use	General English, books, databases	Mathematics, science, economics
Correct?	Yes	Yes
Meaning	Usually plural of index	Usually plural of index
Example	Book indexes	Economic indices

When Should You Use Each?

Use indexes when you are writing in general English or talking about organized lists.

Examples:

• Book indexes
• Database indexes
• Library indexes
• Website indexes
• Search indexes

Use indices when writing in a field where the term is traditionally preferred.

Examples:

• Stock market indices
• Economic indices
• Mathematical indices
• Statistical indices
• Scientific indices

However, these are preferences, not absolute grammatical rules. You may encounter indexes and indices in both general and technical writing.

10+ Examples of Indexes and Indices

Examples with “Indexes”

1. The textbook includes detailed indexes.
2. The library uses subject indexes.
3. Search engines maintain huge web indexes.
4. The database contains several useful indexes.
5. I checked the book’s indexes for the author’s name.
6. The publisher created separate indexes for each volume.

Examples with “Indices”

7. The researchers calculated several statistical indices.
8. Economists monitor different economic indices.
9. The scientist compared environmental indices.
10. Several mathematical indices appear in the formula.
11. Financial analysts study market indices.
12. The report contains multiple performance indices.

Common Mistakes

Mistake 1: Thinking “indices” is always more correct

It isn’t. Indexes is a perfectly standard plural.

❌ The book’s indexes are wrong because “indices” is the proper plural.

✅ The book’s indexes are easy to use.

Mistake 2: Thinking “indexes” is never used in technical writing

Technical writers can use indexes, too. The preferred form depends on the field, publication, and intended meaning.

Mistake 3: Using “indexs”

The plural is not indexs.

❌ indexs
✅ indexes
✅ indices

Mistake 4: Assuming the two forms always have different meanings

Usually, indexes and indices are simply two plural forms of the same word. Context determines which sounds more natural.

Memory Trick

A simple way to remember the difference is:

Indexes = general English
Indices = often technical or academic

Think:

This is a useful guideline, but remember that it is not an absolute rule.

American vs British English

There is no simple American-versus-British rule that makes one form correct and the other incorrect.

Both indexes and indices appear in English varieties around the world.

Usage is more strongly influenced by context and subject area than by whether the writer uses American or British English.

For example, a general English writer may naturally choose indexes, while a statistician may prefer indices.

Similar Words

Understanding related words can make the distinction easier.

Index

The singular form.

Indexes

A plural form commonly used in general English.

Indices

A plural form commonly associated with technical or academic contexts.

Indicator

A value or sign that provides information about a condition or trend.

Indicator vs Index

An index often combines or represents information in a systematic way, while an indicator is generally a measure or sign that provides information about something.

Final Verdict

Indexes and indices are both correct. The best choice depends mainly on context and established usage.

For ordinary English, especially when referring to lists in books, databases, or websites, indexes is a natural choice. In mathematics, economics, statistics, and other technical fields, indices is often preferred.

When writing for a particular academic or professional audience, follow the terminology commonly used in that field and remain consistent.

FAQs:

Is “indexes” grammatically correct?

Yes. Indexes is a standard and correct plural form of index. It is especially common in general English and when referring to lists in books, databases, and other information systems.

Is “indices” grammatically correct?

Yes. Indices is also a correct plural form of index. It is particularly common in mathematics, statistics, economics, science, and other technical contexts.

Which is correct, indexes or indices?

Both are correct. Indexes is often more natural in general English, while indices is frequently preferred in technical and academic contexts.

What is the plural of index in mathematics?

Indices is traditionally common in mathematics, although indexes may also be encountered depending on the specific terminology and publication.

What is the plural of index in a book?

Indexes is a very common choice when referring to the lists at the back of books.

Is “indexs” a correct plural?

No. The correct plural forms are indexes and indices.

Is indices singular or plural?

Indices is plural. Its singular form is index.

Can I use indexes in academic writing?

Yes. Indexes is grammatically correct in academic writing. However, check the terminology normally used in your specific discipline or publication.

Remember This

Index → singular
Indexes → common general-English plural
Indices → traditional plural, often preferred in technical fields

Both indexes and indices are correct. Context determines which one is the better fit.

Understanding this distinction helps you write naturally without treating one plural as universally correct and the other as incorrect.

Conclusion

The difference between indexes and indices is mainly a matter of usage and context. Both are legitimate plural forms of index, so choosing one does not automatically make your sentence grammatically right or wrong. Indexes is widely used in everyday English, especially for lists and databases, while indices is especially familiar in mathematics, economics, statistics, and science.

If you are writing about a book or information list, indexes is usually a straightforward choice. If you are discussing mathematical or scientific measurements, indices may fit the established terminology better. When in doubt, consider your audience, subject, and the terminology commonly used in that field.