Radius is an irregular noun. Its plural form (radii) must be memorized as it does not follow standard rules.
Words that follow the same irregular pattern as radius:
| Singular | Plural | Pattern |
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Radius is irregular. Its plural (radii) must be memorized.
Use plural verbs: "The radii are ready" (not "is ready").
Yes. The plural radii is standard across all English variants.
The word radius is an irregular noun in English. Unlike regular nouns that simply add -s, radius changes to radii in the plural form. This irregular pattern must be memorized as it does not follow the standard rules of English pluralization.
Irregular plurals like radius/radii often trace back to Old English, Latin, or Greek origins. These languages had different ways of forming plurals, and those patterns were preserved as the words entered the English language. While there is no formula to predict irregular plurals, the most common ones are used frequently enough to become natural with practice.
When using radii in writing, remember that it functions as a plural noun in sentences. It pairs with plural verbs and pronouns. For example: "The radii are ready" (not "is ready"). This subject-verb agreement is essential for grammatical correctness.