Irregular Plurals: The Complete Guide

Reviewed by the Grammarlyzer Editorial Team · Our Review Standards · Published January 22, 2026 · Updated June 13, 2026

Master Words That Don't Follow the Rules

Quick Answer

Irregular plurals don't follow the standard -s/-es pattern.

Vowel change: man → men, woman → women, tooth → teeth

-en ending: child → children, ox → oxen

Same form: sheep → sheep, fish → fish, deer → deer

Latin/Greek: cactus → cacti, phenomenon → phenomena

Memory Trick: Irregular plurals change form—memorize the most common ones.

🔑 Key Takeaway

Irregular plurals must be memorized—look for vowel changes, -en endings, or no change at all.

The Main Irregular-Plural Patterns

Irregular plurals fall into a handful of predictable groups. Learn the group and you can usually predict the plural — the only true memorize-it cases are the zero-change and foreign-origin words.

Pattern How it forms Examples
Vowel change internal vowel shifts man→men, foot→feet, tooth→teeth, mouse→mice
-en ending old plural suffix child→children, ox→oxen
No change singular = plural sheep, deer, fish, series, species
-f → -ves f/fe becomes ves leaf→leaves, knife→knives, life→lives
Latin / Greek keeps original ending cactus→cacti, criterion→criteria, analysis→analyses

Common Mistakes

❌ Incorrect:

The selection criterias are strict.

✓ Correct:

The selection criteria are strict.

"Criteria" is already plural (singular: criterion). Adding -s double-pluralizes it. The same trap hits "phenomena" and "data."
❌ Incorrect:

There were three childs in the room.

✓ Correct:

There were three children in the room.

"Child" takes the old -en plural, not -s. "Childs" and "childrens" are both incorrect.
❌ Incorrect:

The dentist checked my tooths.

✓ Correct:

The dentist checked my teeth.

"Tooth" is a vowel-change plural (tooth→teeth), like foot→feet and goose→geese. There's no -s form.
❌ Incorrect:

We caught several fishes in the lake.

✓ Correct:

We caught several fish in the lake.

"Fish" is a zero-change plural for multiple fish. Use "fishes" only when you mean multiple species ("the fishes of the Amazon").

🎯 Test Your Knowledge

1. The plural of "child" is ___.

2. Two important ___ guided the decision.

3. The table is six ___ long.

4. The chef sharpened all the ___.

5. The farmer counted forty ___.

See It Live: Check a Sentence With Our Engine

Don't just trust the rule—test it. The grammar engine below checks your text directly in your browser. The starter sentence has a double-pluralized word—fix it, or paste your own to watch the engine react.

The correct version is: The children lost two of their baby teeth. "Children" is already plural (no extra -s), and "tooth" becomes "teeth" by vowel change.

Honest limits: the engine handles the rule-bound errors well, but with irregular plurals, the call often comes down to rhythm, emphasis, and meaning. Treat the check as a first pass, then make the editorial decision yourself.

Frequently Asked Questions

What is the plural of "person"?

The standard plural of "person" is "people": one person, two people. "Persons" also exists but is now mostly limited to legal or formal language, as in "persons of interest."

Why don't "sheep" and "fish" change in the plural?

Some nouns, especially animals, keep the same form in singular and plural: one sheep, two sheep; one deer, two deer. These zero-change plurals are a survival from Old English. Note that "fishes" is used when referring to multiple species.

Is the plural of "mouse" always "mice"?

For the animal, yes: one mouse, two mice. But for a computer mouse, both "mice" and "mouses" appear; many style guides and dictionaries accept "mouses" or "mouse devices" for the hardware sense.

Real-World Examples

📏 Vowel Change:

man → men, woman → women, tooth → teeth, foot → feet, mouse → mice

Old English umlaut plurals — internal vowel change
📏 -en Plurals:

child → children, ox → oxen

Surviving Old English -en plural endings
📏 Identical:

sheep → sheep, fish → fish, deer → deer

Zero-change plurals — same form for singular and plural
📏 Latin/Greek:

cactus → cacti, analysis → analyses, criterion → criteria, phenomenon → phenomena

Foreign-origin words often keep their original plural forms
❌ Common Error:

The criterias for selection are strict.

Wrong: 'criteria' is already plural. Singular: 'criterion.' Never add -s to 'criteria.'
❌ Common Error:

One phenomena was observed.

Wrong: 'phenomena' is plural. Singular: 'phenomenon.' One phenomenon WAS observed.

Why Do People Confuse Them?

English borrowed words from Latin, Greek, French, and other languages, each with its own pluralization rules. Native speakers often don't realize that 'criteria,' 'phenomena,' and 'data' are already plural forms. Additionally, some irregular plurals are regularizing over time ('cactuses' is now accepted alongside 'cacti').

For more practice, see Subject and Fewer vs Less.

Related Articles

Irregular Plurals in Reader-Facing Drafts

In business writing, irregular plural errors surface most often in technical documents, data reports, and presentations where writers are focused on content rather than form. Mistakes like "datas," "criterias," or "phenomenons" are particularly conspicuous in professional settings because they typically appear in high-stakes contexts — board presentations, white papers, investor updates — where attention to detail signals general competence. Words borrowed from Latin and Greek retain their classical plural forms in formal English: "data" is already plural (singular "datum"), "criteria" is plural (singular "criterion"), and "phenomena" is plural (singular "phenomenon"). Using these words correctly marks a writer as someone who commands both their subject matter and the language used to discuss it.

Academic writing demands precise handling of irregular plurals because many discipline-specific technical terms derive from Latin and Greek roots with non-standard pluralization. In biology, "bacterium" becomes "bacteria" and "alga" becomes "algae." In medicine, "diagnosis" becomes "diagnoses" and "prognosis" becomes "prognoses." In economics, "index" can become "indices" or "indexes" depending on context. Style guides for scientific journals routinely include notes on these forms, and peer reviewers will flag incorrect plurals as evidence of insufficient familiarity with the field's conventions. For students and researchers working across disciplines, it is worth consulting a field-specific glossary whenever an unfamiliar technical plural is needed.

The most frequent error patterns fall into three categories. First, hypercorrection: writers who know that some Latin plurals end in "-i" overcorrect non-Latin words (writing "octopi" instead of "octopuses" or "cactuses," since "octopus" is Greek, not Latin). Second, treating collective irregular nouns as singular or plural inconsistently: "staff," "team," and "data" shift between singular and plural treatment depending on whether British or American conventions are followed, creating inconsistency within a single document. Third, using the singular form in place of the plural: writing "criterion" when "criteria" is needed, or "medium" when "media" is required by context.

The Irregular Plural Strategy

When uncertain, look up the word in a current dictionary rather than guessing from the pattern of similar-sounding words. English irregular plurals do not follow a single consistent rule, so pattern-matching often leads to hypercorrection. When a word has two accepted plural forms (indexes/indices), choose the one that matches your style guide or your discipline's convention.

Grammar Questions About Irregular Plurals

Is "data" singular or plural?

Grammatically, "data" is the plural of the Latin "datum," and treating it as plural — "the data show," "the data were collected" — is standard in scientific and academic writing. However, in general business and journalistic writing, "data" is increasingly treated as a mass noun (like "information"), taking singular verbs: "the data shows," "this data is useful." Many style guides now accept both uses depending on context. The key is consistency: choose one treatment and maintain it throughout a document. Formal scientific papers should still use the plural in most cases, while business reports may use either.

What is the correct plural of "criterion"?

The correct plural of "criterion" is "criteria." This is one of the most commonly misused irregular plurals in academic and professional writing, with many writers using "criteria" as a singular noun ("this criteria is important") or adding an -s to make "criterias," which is not a standard English form. When you need the singular, use "criterion": "One criterion for selection is fluency." When you need the plural, use "criteria": "The selection criteria include fluency, experience, and availability." The word follows the same Latin second-declension pattern as "datum/data" and "bacterium/bacteria."

Is "octopi" the correct plural of "octopus"?

Not strictly. "Octopi" applies a Latin plural ending to a word that comes from Greek, not Latin. The historically correct plural based on the Greek origin would be "octopodes," though this is rare in practice. Most current dictionaries accept three plural forms: "octopuses" (the most common and natural-sounding in English), "octopi" (widely used and understood, though technically a hypercorrection), and "octopodes" (linguistically accurate but pedantic in everyday contexts). For professional and academic writing, "octopuses" is the safest choice. The broader lesson: not every word ending in "-us" takes a Latin "-i" plural.

Why do some words have two accepted plural forms?

Many English words have two accepted plurals because the language absorbed their classical forms while also generating native English plurals over time. "Index" can be "indices" (Latin, used in mathematics and formal contexts) or "indexes" (English, used for book indexes and databases). "Appendix" can be "appendices" (classical, preferred in academic writing) or "appendixes" (English, used in general contexts). "Formula" can be "formulae" (Latin, used in science) or "formulas" (English, used in everyday contexts). When both forms exist, discipline and register guide the choice — science and law tend toward classical forms, while business and journalism prefer the anglicized versions.

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