In English, there are hundreds of nouns that don’t follow the standard rules for pluralization. There are no easy ways to remember them, so they generally have to be memorized.
Some of the rarer irregular plurals are often misused, leading to the creation of variant forms, which usually encounter resistance at first but eventually make it into the language. For example, the plural of formula has traditionally been formulae, but formulas is now far more common, and few would call it incorrect.
Latinate plurals
Some nouns have retained their plural forms from Latin and other languages:
| singular | plural |
| addendum | addenda |
| alga | algae |
| alumnus | alumni |
| amoeba | amoebae |
| antenna | antennae |
| bacterium | bacteria |
| cactus | cacti |
| curriculum | curricula |
| datum | data |
| fungus | fungi |
| genus | genera |
| larva | larvae |
| memorandum | memoranda |
| stimulus | stimuli |
| syllabus | syllabi |
| vertebra | vertebrae |
-o becoming –oes
Most nouns ending in –o are pluralized by adding an –s, but some take –es instead—for example:
| singular | plural |
| echo | echoes |
| embargo | embargoes |
| hero | heroes |
| potato | potatoes |
| tomato | tomatoes |
| torpedo | torpedoes |
| veto | vetoes |
Changing vowel sounds
Some nouns change their central vowel sounds when pluralized—for example:
| singular | plural |
| foot | feet |
| goose | geese |
| louse | lice |
| man | men |
| mouse | mice |
| tooth | teeth |
| woman | women |
-f becoming –ves
Some nouns ending in –f are pluralized by dropping the –f and adding –ves:
| singular | plural |
| calf | calves |
| elf | elves |
| half | halves |
| hoof | hooves |
| knife | knives |
| leaf | leaves |
| life | lives |
| loaf | loaves |
| self | selves |
| shelf | shelves |
| thief | thieves |
| wife | wives |
| wolf | wolves |
-is becoming –es
Some nouns ending in –is are pluralized by dropping the –is and adding –es:
| singular | plural |
| axis | axes |
| analysis | analyses |
| basis | bases |
| crisis | crises |
| diagnosis | diagnoses |
| ellipsis | ellipses |
| emphasis | emphases |
| hypothesis | hypotheses |
| neurosis | neuroses |
| oasis | oases |
| paralysis | paralyses |
| parenthesis | parentheses |
| thesis | theses |
-ix becoming –ices
Some nouns ending in –ix are pluralized by dropping the –ix and adding –ices:
| singular | plural |
| appendix | appendices |
| index | indices |
| matrix | matrices |
Unchanged plurals
A few nouns are unchanged in their plural forms:
| singular | plural |
| barracks | barracks |
| deer | deer |
| fish | fish |
| gallows | gallows |
| means | means |
| offspring | offspring |
| series | series |
| sheep | sheep |
| species | species |