Plural of Radius

by Craig Shrives

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

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. correct tick
  • (It is acceptable to use radiuses, but radii is more common.)
  • Atomic radii are not precisely defined but are widely in understanding molecular structures. correct tick

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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
add s yetis
Noun Ending s, sh, ch, x or z radius
add es radiuses
Nouns ending [consonant] o mango
add either s or es
(There are no rules for this - you have to know.)
mangoes or mangos
Nouns ending [vowel] o patio
add s patios
Nouns ending [consonant] y story
change the y to an i and add es stories
Nouns ending [vowel] y storey (small British flag)
add s storeys
Nouns ending f or fe dwarf
ves and/or s
(There are no rules - you have to know.)
Exceptions man
some nouns undergo a vowel or letters change men
More exceptions radius
some nouns do not change at all or adopt a foreign ending (typically Latin) radii
Read more about English spelling rules.

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]

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See Also

Unusual pluralsPlural forming table Quirks with forming plurals Forming the plurals of abbreviations Forming the plurals of compound nouns