Indexes and indices are both acceptable plurals of the noun index, and they appear about equally often. Indices is standard in English varieties from outside North America, while indexes is more common in American and Canadian English. Meanwhile, indices is generally more common in mathematical and technical contexts, while indexes is common in informal contexts.
Don’t listen to anyone who says either form is wrong. Both forms have been in English many centuries (and, though indexes is preferred in American English, it predates the United States). And while it’s true that indices is the correct plural of index in Latin, index is an English word when English-speakers use it, so we can pluralize it in the manner of our own language.
Examples
Most major American publications prefer indexes as the plural of index—for example:
A quick tour around the world’s stock indexes will reveal the extent of Mr. Market’s New Year rethink. [Wall Street Journal]
This year is looking tougher, though. Asian stock indexes are off to a rocky start with increasing concerns over rising inflation and other macroeconomic conditions. [New York Times]
Indexes in idle mode: Stock rally fed by Europe deal slows [USA Today]
Outside the U.S. and Canada, indices is preferred—for example:
Despite the fact we have major challenges of poverty, in all the indices of community engagement we are thriving. [Guardian]
Of the 45 indices that make up the MSCI World Index, 36 recorded declines of more than 20 per cent . . . [Irish Times]
For the average direct investor or trader, the market and the indices that represent it are almost completely irrelevant. [Sydney Morning Herald]
