The Most Common Irregular Plural Nouns in English

There are no easy rules, unfortunately, for irregular plurals in English. They simply have to be learnt and remembered.
(S. Curtis and M. Manser, The Penguin Writer's Manual, 2002)

Most English nouns form their plural by adding either -s (books, bands, bells) or -es (boxes, bunches, batches). These plural forms are said to follow a regular pattern.

But not all nouns conform to this standard pattern. In fact, some of the most common English nouns have irregular plural forms--such as woman/women and child/children.

(The reasons for this are briefly discussed in the article Plural Forms of English Nouns.) In addition, several nouns have alternative plurals, one regular and the other irregular.

In regard to these alternative forms, there are no strict rules to guide our use of them:

People have to learn which form to use as they meet the words for the first time, and must become aware of variations in usage. When there is a choice, the classical [irregular] plural is usually the more technical, learned, or formal, as in the case of formulas vs formulae or curriculums vs curricula . Sometimes, alternative plurals have even developed different senses, as in the cases of (spirit) mediums vs (mass) media , or appendixes (in bodies or books) vs appendices (only in books).
(David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge University Press, 2003)

As you'll see in the list that follows, many words with irregular plurals are loanwords that have kept their foreign plural forms (or at least held on to those forms as alternatives to regular English plurals).

List of 100 Irregular Plural Nouns in English

In the list below, you'll find singular noun forms in the left column and the corresponding plural forms in the right column. When a noun has more than one plural form, the irregular one appears first, though that doesn't necessarily mean that the irregular form is more widely accepted than the regular form.

SOURCES: The plural forms in this list are recognized by Merriam-Webster's Collegiate Dictionary (2003) and The American Heritage Dictionary of the English Language (2011).

addendum	addenda or addendums
aircraft	aircraft
alumna	alumnae
alumnus	alumni
analysis	analyses
antenna	antennae or antennas
antithesis	antitheses
apex	apices or apexes
appendix	appendices or appendixes
axis	axes

bacillus	bacilli
bacterium	bacteria
basis	bases
beau	beaux or beaus
bison	bison
bureau	bureaux or bureaus

cactus	cacti or cactus or cactuses
château	châteaux or châteaus
child	children
codex	codices
concerto	concerti or concertos
corpus	corpora
crisis	crises
criterion	criteria or criterions
curriculum	curricula or curriculums

datum	data
deer	deer or deers
diagnosis	diagnoses
die	dice or dies
dwarf	dwarves or dwarfs

ellipsis	ellipses
erratum	errata

faux pas	faux pas
fez	fezzes or fezes
fish	fish or fishes
focus	foci or focuses
foot	feet or foot
formula	formulae or formulas
fungus	fungi or funguses

genus	genera or genuses
goose	geese
graffito	graffiti
grouse	grouse or grouses

half	halves
hoof	hooves or hoofs
hypothesis	hypotheses

index	indices or indexes

larva	larvae or larvas
libretto	libretti or librettos
loaf	loaves
locus	loci
louse	lice

man	men
matrix	matrices or matrixes
medium	media or mediums
memorandum	memoranda or memorandums
minutia	minutiae
moose	moose
mouse	mice

nebula	nebulae or nebulas
nucleus	nuclei or nucleuses

oasis	oases
offspring	offspring or offsprings
opus	opera or opuses
ovum	ova
ox	oxen or ox

parenthesis	parentheses
phenomenon	phenomena or phenomenons
phylum	phyla
prognosis	prognoses

quiz	quizzes

radius	radii or radiuses
referendum	referenda or referendums

salmon	salmon or salmons
scarf	scarves or scarfs
self	selves
series	series
sheep	sheep
shrimp	shrimp or shrimps
species	species
stimulus	stimuli
stratum	strata
swine	swine
syllabus	syllabi or syllabuses
symposium	symposia or symposiums
synopsis	synopses

tableau	tableaux or tableaus
thesis	theses
thief	thieves
tooth	teeth
trout	trout or trouts
tuna	tuna or tunas

vertebra	vertebrae or vertebras
vertex	vertices or vertexes
vita	vitae
vortex	vortices or vortexes

wharf	wharves or wharfs
wife	wives
wolf	wolves
woman	women