Regular and Irregular Plurals
Most countable nouns in English have a singular and a plural form (cat/cats, child/children, story/stories). The plural may be either regular or irregular.
Regular plurals are formed by adding -s, -es, or -ies to the singular (e.g., girls, viruses, stories). Irregular plurals also often follow a pattern, originating sometimes in the parent language or rules of older forms of English (children, criteria, oases, geese, mice, indices).
Some nouns have both regular and irregular plurals (indexes/indices, formulas/formulae), one or other of which may be preferred in British or American English.
Standard dictionaries (such as Cambridge or Collins for British English and Merriam-Webster for American) include in their word entries the plural forms of countable nouns. For words with more than one plural form, the preferred spelling (syllabi/syllabuses, matrices/matrixes, flamingos/flamingoes) is that which is listed first.
You should find it easier to remember the plural forms of words once you familiarize yourself with the following common patterns by which regular and irregular plurals are formed in English. We also discuss some exceptions and interesting examples.
Regular plurals
In general, plurals of nouns are formed by adding -s or -es to the singular form.
Plurals formed by adding -s
| Singular | Plural |
|---|---|
| girl | girls |
| house | houses |
| root | roots |
| roof | roofs |
| shoe | shoes |
| piano | pianos |
| argument | arguments |
Plurals formed by adding -es
Regular plurals of words ending in -s, -x, -z, -ch, or -sh are formed by adding -es.
| Singular | Plural |
|---|---|
| virus | viruses |
| ignoramus | ignoramuses |
| loss | losses |
| glass | glasses |
| ax | axes |
| box | boxes |
| buzz | buzzes |
| blitz | blitzes |
| church | churches |
| watch | watches |
| dish | dishes |
| rash | rashes |
The plural of fish is usually just fish.
- All the fish in Farley’s aquarium have died.
When referring to different species of fish, however, scientists may use the word fishes.
- A study was conducted on nine kinds of fishes in Lake Tupututu.
When a word ends in -ch but is pronounced with a k sound rather than a ch sound, the plural takes an -s instead of an -es.
| Singular | Plural |
|---|---|
| stomach | stomachs |
| monarch | monarchs |
These plurals are similar to duck/ducks and lock/locks.
Plurals formed by changing -y to -ies
For words ending in a consonant followed by y, form the plural by changing -y to -ies.
| Singular | Plural |
|---|---|
| baby | babies |
| berry | berries |
| duty | duties |
| lady | ladies |
| army | armies |
| battery | batteries |
Plurals formed by adding -es to -o
For nouns ending in a consonant followed by -o, plurals are often formed by adding -es.
| Singular | Plural |
|---|---|
| volcano | volcanoes |
| potato | potatoes |
| tomato | tomatoes |
| mango | mangoes |
However, there are exceptions, especially for nouns of foreign origin.
| Singular | Plural |
|---|---|
| piano | pianos |
| studio | studios |
| photo | photos |
| radio | radios |
Irregular plurals
Some nouns don’t follow the regular pattern of forming plurals simply by adding -s or -es. These comprise words that have come into English from a foreign language or that follow older English rules. Such plural nouns are termed “irregular.”
| Singular | Plural |
|---|---|
| woman | women |
| child | children |
| foot | feet |
| tooth | teeth |
| goose | geese |
| mouse | mice |
| louse | lice |
| ox | oxen |
| die | dice |
When the word mouse refers to the small device used to control the movements of a cursor on a computer screen, its plural can be either mice or mouses. When mouse refers to a little animal, though, the plural is always mice.
- Vertical mice/mouses may be helpful if you have carpal tunnel syndrome.
- Do cats eat mice?
Plurals formed by changing -um or -on to -a
| Singular | Plural |
|---|---|
| criterion | criteria |
| bacterium | bacteria |
| curriculum | curricula |
| equilibrium | equilibria |
| medium | media |
| paramecium | paramecia |
| phenomenon | phenomena |
Criteria and bacteria are already plurals, so don’t add -s to them.
- We used multiple criteria/
criteriasto evaluate the results. - Probiotics replenish gut bacteria/
bacterias.
Also, remember that the singular forms are criterion and bacterium.
- I have just one criterion/
criteria. - We found a single bacterium/
bacteriain the entire sample.
Plurals formed by changing -us to -i
| Singular | Plural |
|---|---|
| alumnus | alumni |
| bacillus | bacilli |
| cactus | cacti |
| focus | foci |
| fungus | fungi |
| locus | loci |
| nucleus | nuclei |
| radius | radii |
| stimulus | stimuli |
Some of these words have alternative plural forms (such as cactuses and focuses), no longer considered wrong but not yet preferred.
Plurals formed by changing -a to -ae
| Singular | Plural |
|---|---|
| alga | algae |
| alumna | alumnae |
| amoeba | amoebae |
| larva | larvae |
| formula | formulae |
| vertebra | vertebrae |
These words also often have alternative plural forms (such as larvas, formulas, and vertebras), common in American English.
An alumnus is a school’s former pupil, especially a male person. An alumna is a former pupil who is female. The plural of alumnus is alumni, and that of alumna is alumnae. The plural form alumni, however, is often used to refer to all former pupils, both male and female.
Plurals formed by changing -ix or -ex to -ices
| Singular | Plural |
|---|---|
| appendix | appendices |
| matrix | matrices |
| cervix | cervices |
| index | indices |
| vertex | vertices |
Such words also have alternative plural forms (such as matrixes, vertexes, and appendixes), often used in American English. In particular, when used in the anatomical sense, appendixes is preferred.
The final syllable (-ces) of the plural is pronounced with the long E sound: for example, matrices is pronounced MAY-tri-seez.
Plurals formed by changing -f or -fe to -ves
| Singular | Plural |
|---|---|
| knife | knives |
| hoof | hooves |
| wife | wives |
| life | lives |
| leaf | leaves |
| loaf | loaves |
| calf | calves |
| half | halves |
| dwarf | dwarves |
| shelf | shelves |
| scarf | scarves |
| thief | thieves |
| wolf | wolves |
Plurals formed by changing -is to -es
| Singular | Plural |
|---|---|
| analysis | analyses |
| axis | axes |
| basis | bases |
| crisis | crises |
| ellipsis | ellipses |
| emphasis | emphases |
| hypothesis | hypotheses |
| oasis | oases |
| thesis | theses |
| parenthesis | parentheses |
The final syllable (-ses) of the plural is pronounced with the long E sound: for example, crises is pronounced KREYE-seez.
The plural of base is bases and that of basis is also bases; however, the two plurals are pronounced differently. The first is pronounced BAY-səz, and the second, BAY-seez. Similarly, the plural of both ax and axis is axes, but the plural of the heavy-edged cutting tool is pronounced with a short E sound (ax/axes or AK-səz), while that of the straight line around which a geometric figure rotates is pronounced with a long E sound (axis/axes or AK-seez).
No change in plural spelling
Some words are spelled the same in their singular and plural forms.
| Singular | Plural |
|---|---|
| moose | moose |
| deer | deer |
| sheep | sheep |
| aircraft | aircraft |
| species | species |
The plural of aircraft is aircraft, not aircrafts. Don’t add an s to form the plural.
- We couldn’t hear ourselves think, what with all the aircraft/
aircraftsflying overhead. - Farley can crash all types of aircraft/
aircrafts: airplanes, helicopters, gliders, hot air balloons, airships, etc.
Nouns that are always plural
Some nouns are always used in the plural form.
- These scissors aren’t sharp enough to cut paper.
- Poco bought himself a new pair of trousers for the alien invasion.
- Has Farley lost his pants again?
- For her trip, Rita packed her binoculars and goggles as well as a spare pair of spectacles.
- Why must we always wear clothes?
- Thanks to everyone who follows us on Bluesky.
Nouns with both regular and irregular plurals
Many words retain the plural form from the parent language but have an alternative plural based on English rules (adding -s or -es). Here are some examples.
| Singular | Plural |
|---|---|
| octopus | octopodes/octopuses/octopi |
| cactus | cacti/cactuses |
| virtuoso | virtuosi/virtuosos |
| chateau | chateaux/chateaus |
| hobo | hoboes/hobos |
| flamingo | flamingoes/flamingos |
| volcano | volcanoes/volcanos |
| formula | formulae/formulas |
| syllabus | syllabi/syllabuses |
| matrix | matrices/matrixes |
| index | indices/indexes |
| appendix | appendices/appendixes |
| equilibrium | equilibria/equilibriums |
| millennium | millennia/millenniums |
For some words, one or other of the plural forms is preferred in British or American English. Standard dictionaries will list the preferred alternative first.
Plurals of compound nouns
Compound nouns, which are formed by combining two words, usually follow the plural spelling of the original word.
| Singular | Regular plural |
|---|---|
| workhouse | workhouses |
| schoolgirl | schoolgirls |
| gumboot | gumboots |
| bedroom | bedrooms |
| wristwatch | wristwatches |
| Singular | Irregular plural |
|---|---|
| gunman | gunmen |
| penknife | penknives |
| bookshelf | bookshelves |
| lactobacillus | lactobacilli |
To form plurals of open and hyphenated compounds, check which word is actually being pluralized. For example, the plural of sister-in-law is sisters-in-law, not sister-in-laws. (It is the sisters who are many, not the laws.)
| Compound singular | Compound plural |
|---|---|
| daughter-in-law | daughters-in-law |
| runner-up | runners-up |
| passerby | passersby |
| lady-in-waiting | ladies-in-waiting |
| attorney-at-law | attorneys-at-law |
| minister-in-charge | ministers-in-charge |
| Compound singular | Compound plural |
|---|---|
| bystander | bystanders |
| carbon copy | carbon copies |
| run-in | run-ins |
| look-alike | look-alikes |
| home run | home runs |
The plural of professor emeritus is professors emeriti. This comes from Latin, where, unlike in English, if the noun is plural (professor → professors), the adjective must also be pluralized (emeritus→ emeriti).