Irregular Plural Nouns Explained: Rules and Examples

English is an interesting language: for someone born into an English-speaking home, it can seem pretty straightforward. But foreigners wishing to learn English as a second language need to memorize many rules—as well as the many exceptions to those rules! 

For instance, for phonics, you may learn how to sound out each letter. But when these letter sounds occur in different words, they may be sounded out totally different, as in the case of sight words. 

In terms of grammar, you also need to learn how singular words become plural. In English, regular nouns become plural by adding -s or -es to the end of the word. For example: 

Singular – Plural

• cat – cats
• dog – dogs
• house – houses
• tree – trees
• ball – balls

But the language also includes numerous words that do not follow this rule. These are known as irregular plural nouns. 

What Are Irregular Plural Nouns? 

An irregular plural noun refers simply to nouns that do not form their plural by adding -s or -es. A very common example is the word man: you don’t form the plural by making it mans; instead, the plural form is men. 

Examples of Irregular Plural Nouns 

Irregular plural nouns come in several forms: 

1. Nouns that end in -f or -fe 

To make the plural form of a word that ends in -f, change the f to v and add -es.

If the word ends in -fe, change –fe to –ve and add –s. 

Singular – Plural

• calf – calves
• hoof – hooves
• knife – knives
• leaf – leaves
• life – lives
• sheaf – sheaves
• wife – wives
• wolf – wolves 

Some of the exceptions to this rule include: 

Singular – Plural 

• roof – roofs
• proof – proofs 

2. Nouns that change vowels for their plural form 

Many nouns in English form their plural by changing the vowels, usually from oo to ee, and an to en. 

Singular – Plural

• foot – feet
• goose – geese
• man – men
• tooth – teeth
• woman – women 

The challenge is that you really need to familiarize yourself with the words, because the rule does not apply to all words with oo in it. For example, these words have regular plural forms: 

Singular – Plural 

• boot – boots
• loop – loops
• hoop – hoops

3. Nouns that change substantially 

The following words change their whole spelling to form their plural: 

Singular – Plural 

• die – dice 
• louse – lice
• mouse – mice
• ox – oxen
• child – children

4. Irregular nouns borrowed from Latin or Greek and ending in –us 

We borrow many English words from Latin or Greek, especially those used in scientific or mathematical concepts. These words usually keep their Latin or Greek plural forms, particularly in science and math contexts. Some of them also come with anglicized plural forms that are also widely accepted.

To make the plural form of words ending in -us, change -us to –is. To form the anglicized plural form of these words, simply add -es; you can use these in informal settings. 

Singular – Plural – *Anglicized (also accepted for informal settings) 

• alumnus – alumni
• bacillus – bacilli
• cactus – cacti 
• focus – foci – focuses 
• fungus – fungi 
• hippopotamus – hippopotami – hippopotamuses
• nucleus – nuclei
• octopus – octopi* – octopuses
• radius – radii – radiuses
• stimulus – stimuli 
• syllabus – syllabi 

*For octopus, strict grammarians argue about whether the plural should be octopi (which is the plural for Latin words ending in –us) or octopodes (the plural for Greek words ending in –us, since octopus is of Greek origin. To be safe, you can use octopuses, which is accepted by major dictionaries like Merriam Webster.

5. Irregular nouns with Latin or Greek origins ending in –a

For nouns borrowed from Latin or Greek and ending in -a, add an e to form its plural. 

Singular (-a) – Plural (-ae)

• alumna – alumnae
• amoeba – amoebae
• antenna – antennae
• formula – formulae
• larva – larvae 

6. Irregular nouns borrowed from Latin or Greek and ending in -is 

For words with Latin or Greek origins and ending in -is, we form their plural by changing -is to -es. 

Singular (-is) – Plural (-es)

• analysis – analyses
• axis – axes (Note that this word is also the plural form of the word ax or axe.) 
• basis – bases 
• crisis – crises 
• diagnosis – diagnoses
• ellipsis – ellipses
• hypothesis – hypotheses
• oasis – oases
• parenthesis – parentheses
• thesis – theses

7. Irregular nouns borrowed from Greek and ending in –on

To form the plural of these words with Greek origins, change the ending from -on to -a. 

Singular (-on) – Plural (-a)

• criterion – criteria
• phenomenon – phenomena

8. Irregular nouns borrowed from Greek or Latin ending in –um

To form the plural of words ending in -um, remove -um and replace it with -a. Note that for many of these words, people tend to be more familiar with their plural form rather than their singular.  

Singular (-um) – Plural (-a)

• addendum – addenda 
• bacterium – bacteria
• curriculum – curricula
• datum – data
• erratum – errata 
• medium – media 
• memorandum – memoranda
• ovum – ova
• phylum – phyla
• stratum – strata

9. Irregular nouns borrowed from Greek or Latin ending in -ix or -ex 

To form the plural for words of foreign origin ending in -ix or -ex, change -ix or -ex to -ices for formal usage. Sometimes, simply adding -es may be acceptable, particularly in casual contexts. 

Singular (-ix) – Plural (-ices) – Informal (-es) 

• appendix – appendices – appendixes (medical context)
• codex – codices 
• index – indices – indexes
• vortex – vortices – vortexes 

10. Irregular nouns borrowed from French ending in –eau 

For words that have French origins ending in –eau, you can form the plural by adding –x or –s. 

Singular – Plural 

• beau – beaux or beaus
• chateau – chateaux or chateaus 
• tableau – tableaux or tableaus

11. Irregular nouns that retain their spellings for the plural form 

Other nouns remain the same for both the singular and plural.

Singular – Plural 

• aircraft – aircraft 
• cash – cash
• deer – deer
• fish – fish
• series – series 
• sheep – sheep
• species – species

For these words, you will need the context to determine whether the word functions in singular or plural form. For example: 

Tom made a bet with his brother Sweyn about who would catch the most fish. Sweyn caught one big fish, but Tom was able to catch four big fish using his great brain. 

In the example above, we can see that the first word is used in the plural; the second is singular, and the last is again plural. 

Important note on scientific usage: 

In this category, for nouns referring to animals, you may use the regular plural form, such as “fishes” or “sheeps” in a scientific context to refer to more than one species or multiple breeds of the creature. 

Irregular Plural Nouns Quiz 

Test your knowledge of irregular plural nouns by downloading our irregular plural nouns quiz. Write the plural form of the nouns given below. (Note that this list includes both regular and irregular nouns.) 

Singular – Plural 

1. box
2. cash
3. radius
4. leaf
5. copy 
6. bookshelf 
7. focus
8. bacterium 
9. pen
10. goose 
11. phenomenon 
12. mouse 
13. species
14. woman 
15. thesis

Answer Key

Singular – Plural 

1.  box – boxes
2.  cash – cash
3.  radius – radii
4.  leaf – leaves
5.  copy – copies
6.  bookshelf – bookshelves
7.  focus – foci or focuses 
8.  bacterium – bacteria
9.  pen – pens
10. goose – geese
11. phenomenon – phenomena
12. mouse – mice
13. species – species
14. woman – women
15. thesis – theses

Familiarizing Yourself with Irregular Plural Nouns 

One way of knowing the right plural form to use is to memorize the rules and the example words.

But another way is to familiarize yourself with them by reading extensively, especially works such as literary fiction or classic stories. 

Did you find this post helpful? Let us know in the comments below!

 

If you enjoyed this post, then you might also like:

Yen Cabag is the Blog Writer of TCK Publishing. She is also a homeschooling mom, family coach, and speaker for the Charlotte Mason method, an educational philosophy that places great emphasis on classic literature and the masterpieces in art and music. She has also written several books, both fiction and nonfiction. Her passion is to see the next generation of children become lovers of reading and learning in the midst of short attention spans.

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.

Singular
Plural (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

Singular
Plural

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

deer

moose

swine

buffalo

shrimp

trout

deer
when walking in the woods near here.

I have seen severalwhen 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)

axis
axes (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 (-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.