Indexes or Indices – What’s the Difference?

When words have more than one variant, choosing between them can be difficult. Often, there is no hard and fast rule, so writers must rely on context to determine the best word to use.

Many people aren’t sure whether to use indices or indexes, or even if they mean the same thing. This article will shed some light on these confusing terms.

What is the Plural of Index?

In this post, I will show you the difference between indices and indexes. I will use each of the two words in example sentences that will display them in context. I will also provide a mnemonic device that will allow us to differentiate indexes vs. indices.

When to Use Indexes

define indices define indexesWhat does indexes mean? Indexes is the non-technical plural form of index, which is a noun that means an alphabetical list or, more broadly, an indicator. Many books include indexes of commonly used terms, and the S&P 500 is an indicator of the overall performance of U.S. stocks.

Here are a few more examples,

  • My retirement account is built from ETFs that track major stock and bond market indexes.
  • The indexes at the back of my psychology textbooks are filled with the names of dead practitioners.
  • The number of market indexes now exceeds the number of U.S. stocks. Traditional ones such as the S&P 500 are collections of securities weighted by market value, and index funds mimic them as a low-cost way to deliver the market’s performance. –Bloomberg BusinessWeek

For those writers adhering to AP Style, you should use this spelling and this spelling alone, as AP prefers indexes over indices.

As a verb, indexes is the singular third person present conjugation of the verb index, which means to record names in a list or to serve as an indicator.

Here are a few more ways to conjugate this verb,

  • I/we index: first person singular and plural present
  • You index: second person singular and plural present
  • He/she/it indexes: third person singular present
  • They index: third person plural present
  • Indexing: present participle
  • Indexed: simple past

When to Use Indices

what is the plural for indexWhat does indices mean? Indices is a more technical version of the same plural noun. This version more closely approximates the original Latin form. It is common in many mathematical and technical contexts.

  • David Blitzer, managing director and chairman of the index committee at S&P Dow Jones Indices, said the low stock of existing homes for sale — currently about 3.8 months worth of supply at current sales rates — is bolstering the price increases across the board. –CNBC

It is a fully accepted variant, and, indeed, it enjoys an advantage in overall usage in published English.

Indices vs. Indexes:

indexes versus indices

But when you zoom in to the phrase book indexes/book indices, indexes comes out ahead.

definition of indexes definition of indices definition

Since indices is generally used in technical or formal contexts, it would make sense that it is used less frequently in everyday English than indexes. The above charts tell a somewhat mixed story.

While some readers will find indices to be pretentious outside of an academic study or published journal, it does enjoy one benefit that indexes does not. It eliminates any confusion between it and the verb indexes. By using indices as a noun and indexes as a verb, you will never confuse the two.

Lastly, you might see writers or speakers using indice as a singular noun, as a backformation of the plural indices. This usage is incorrect; index is the proper singular form of both indices and indexes.

Indices only functions as a noun. It cannot be used as a verb.

Trick to Remember the Difference

Both of these forms are accepted plural forms for this word. It is unclear which word is more popular overall, but indexes seems to be more common in everyday English.

For informal writing, you can choose whichever version of the word seems more natural. In some mathematical contexts, indices is the preferred form.

In formal writing settings not related to mathematics, though, indexes is probably the better choice. Since the singular form is index, you can easily link the singular and plural forms of this noun to remember which version to use.


Is it indexes or indices? Indexes and indices are two versions of a plural noun that means an indicator or a list of names. Indexes is also a present tense verb, but indices cannot be used that way.

  • Indexes is probably the better choice for formal writing not related to mathematics.
  • Indices is especially common in technical and formal writing.
  • AP Style requires the spelling indexes.

Let style and flow be your guide with these two words.