Plural of Radius

What Is the Plural of Radius?

The Quick Answer

The plural of "radius" is "radii" or "radiuses." Both "radii" and "radiuses" are used in the UK and the US. "Radii" is far more common in published writing and academic papers.

The plural of "radius" is "radii," but "radiuses" is also used. "Radii" is far more common. The word "radiuses" has only recently (since the 1990s) started to feature in published words.

Calculate the radii of these circles from their circumferences.
Atomic radii are not precisely defined but are widely in understanding molecular structures.

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The Standard Rules for Forming the Plurals

The table below shows where "radius" fits within the standard rules for forming the plurals of nouns in English.

Type	Example of Type	Forming the Plural	Plural
Most Nouns	yeti scythe	add s	yetis scythes
Noun Ending s, sh, ch, x or z	radius dress	add es	radiuses dresses
Nouns ending [consonant] o	mango zero tomato	add either s or es (There are no rules for this - you have to know.)	mangoes or mangos zeros tomatoes
Nouns ending [vowel] o	patio ratio	add s	patios ratios
Nouns ending [consonant] y	story penny	change the y to an i and add es	stories pennies
Nouns ending [vowel] y	storey () donkey	add s	storeys donkeys
Nouns ending f or fe	dwarf knife	ves and/or s (There are no rules - you have to know.)	dwarfs knives
Exceptions	man louse	some nouns undergo a vowel or letters change	men lice
More exceptions	radius salmon sheep	some nouns do not change at all or adopt a foreign ending (typically Latin)	radii salmon sheep

Why Is There Confusion over the Plural of Radius?

"Radius" comes from Latin. It literally means "the spoke of wheel." "Radius" is one of those words that has retained its Latin plural (i.e., "radii"). In English, there are many words that come from Latin, but not all of them have retained their Latin plurals. For example, "referendums" and "syllabuses" (the standard plurals) are far more common than "referenda" and "syllabi" (the Latin plurals).

As a result, writers are unsure whether to adopt the standard method for forming the plural of a Latin word or to use the Latin plural.

In the case of "radius," the most common practice by far is to use the Latin plural "radii." [evidence]