Irregular plural nouns are nouns that do not become plural by adding -s or -es, as most nouns in the English language do. You’re probably familiar with many of these already. For example, the plural form of man is men, not mans. The plural form of woman is women, not womans. There are hundreds of irregular plural nouns, and in truth, you must memorize them through reading and speaking. There are, however, some common patterns to look out for.

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The most common irregular plurals

Nouns ending in -f and -fe

To make a plural of a word ending in -f, change the f to a v and add es. Similarly, if a word ends in -fe, change the f to a v and add an s. The result for both types is a plural that ends in -ves. This spelling arose because of the difficulty of pronouncing f and s together in English (an attempt to do this will produce a v sound).

Singular (-f, -fe)Plural (-ves)

knife knives

life lives

wife wives

calf calves

leaf leaves

Exceptions: roofs and proofs (among others).

Nouns ending in -o

Plurals of words ending in -o are usually made by adding -es.

Singular (-o)Plural (-oes)

potato potatoes

tomato tomatoes

hero heroes

torpedo torpedoes

veto vetoes

But of course, there are exceptions. (Aren’t there always?) Some words ending in -o that are borrowed from other languages take only an s to make a plural, such as pianos, cantos, photos, and zeros. Cello, which is an abbreviation of the Italian word violoncello, can be written the traditional way, celli, or the commonly accepted anglicized way, cellos.

Nouns that change vowels

Many English words become plural by changing their vowels, such as oo to ee or an to en.

SingularPlural (vowel change)

foot feet

tooth teeth

goose geese

man men

woman women

Fun fact: The eighteenth-century American dictionary reformer Noah Webster preferred spellings that were closer to their most common pronunciations. Thus, he advocated for the return of the Old English plural wimmen. Wouldn’t that have been convenient?

Irregular nouns that change substantially

For a variety of historical reasons, some words change in spelling substantially when made plural.

louselice

SingularPlural

mouse mice

die dice

ox oxen

child children

person people*

penny pence (in British usage)

Irregular nouns that do not change at all when made plural

Some English nouns are identical in both the singular and the plural forms. Many of these are names for animals.

Singular/ Plural (no change)

sheep

fish

moose

swine

buffalo

shrimp

trout

I have seen several deer when walking in the woods near here.

How many shrimp did you catch?

Aircraft, watercraft, hovercraft, and spacecraft are all the same whether singular or plural.

NASA has made several different types of spacecraft in their fifty-nine-year history.

Plurals of Latin and Greek words

There are certain words we use on a regular basis, especially in mathematical and scientific contexts, that are borrowed from Latin or Greek. Many of these words retain their Latin or Greek plurals in math and science settings. Some of them also have anglicized plural forms that have come into common use.

Nouns ending in -us

To make a word ending in -us plural, change -us to -i. Many plurals of words ending in -us have anglicized versions, formed by simply adding -es. The latter method sounds more natural in informal settings. If there is an anglicized version that is well accepted, this will be noted in the dictionary entry for the word you are using.

Singular (-us)Plural (-i)

focus foci (also focuses)

radius radii (also radiuses)

fungus fungi

nucleus nuclei

cactus cacti

alumnus alumni*

octopus octopuses (or octopi)

hippopotamus hippopotami (or hippopotamuses)

With the double i, radii (pronounced RAY-dee-i) sounds unwieldy, but if you are a mathematician, you probably use it every day. If you are a zoologist, you might say, “Hey, did you see those hippopotami?” but it would sound silly on a casual visit to the zoo. Many people resist the spelling octopuses, but it is perfectly acceptable. In fact, if you put a fine point on it, since octopus is of Greek origin rather than from Latin, theoretically the spelling should be octopodes, not octopi.

Irregular formation of nouns ending in -is

Nouns with an -is ending can be made plural by changing -is to -es. Some people have a hard time remembering that the plural of crisis is crises and the plural of axis is axes, but crisises and axises are incorrect.

Singular (-is)Plural (-es)

axisaxes (this is also the plural of ax and axe)

analysis analyses

crisis crises

thesis theses

Irregular formation of nouns ending in -on

These Greek words change their -on ending to -a.

Singular (-on)Plural (-a)

phenomenon phenomena

criterion criteria

Irregular formation of nouns ending in -um

Words ending in -um shed their -um and replace it with -a to form a plural. The plurals of some of these words are far better known than their singular counterparts.

Singular (-um)Plural (-a)

datum data

memorandum memoranda

bacterium bacteria

stratum strata

curriculum curricula (also curriculums)

Irregular formation of nouns ending in -ix

Nouns ending in -ix are changed to -ices in formal settings, but sometimes -xes is perfectly acceptable.

Singular (-ex, -ix)Plural (-ces, -xes)

index indices (or indexes)

appendix appendices (or appendixes, in a medical context)

vortex vortices (or vortexes)

These rules for irregular plural nouns must simply be memorized, although it is helpful to understand the patterns first in order to master them. We also have information on the Grammarly blog about patterns for regular nouns.

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