Irregular Plural Nouns Header Image

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 – wome

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 – stimul
  • 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 – errat
  • medium – medi
  • 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. 

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