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What’s the Plural of Radius? Radii, Radiuses?

The plural of radius is radii or radiuses. Radius is singular.

Is “radius” plural or singular?

If the plural of cactus is cacti, and octopus is octopi . . . does that mean the plural of radius should be radii? What’s the correct plural of radius?

In fact, radius plural is indeed radii . . . and radiuses (radiuses being the more recent plural version). Why does radius have two accepted plural forms, and which should you choose? Is one plural more correct than the other? Keep on reading to find out.

What does the word radius mean?

The online dictionary defines the word radius as:

A straight line extending from the center of a circle or sphere to the circumference or surface: The radius of a circle is half the diameter.

What’s the plural of radius?

To mention numerous radiuses, use radii or radiuses. The original Latin plural noun form is radii. Like other Latin derived words, radiuses has come to be accepted over time, which is likely due to the ongoing confusion of its proper plural form.

Nouns that end in -us/-i

cactuscacti (or cactuses)
octopusoctopi (or octopuses)
radiusradi (or radiuses)
fungusfungi (or funguses)
alumnusalumni (or almunuses)
syllabussyllabi (or syllabuses)
Latin nouns ending in –us/-i. Chart by Grammarflex.

Radius, singular, used in sentence examples

1. But if you want to breakfast on it just make sure you stay out of the halitotic radius.

2. The action is set in a suburb outside East London when the city has grown into a megalopolis of 30 miles in radius.

3. The radius of circle or arc you can make with this beam compass set is limited only by the ledge you have at hand.

4. The greatest of all versed sines is a diameter, as that of all natural sines is a radius.

5. A useful escape tactic to a prey is to initiate a turn before predator closure and rely on a tight turn radius for escape.

Radiuses/radii, plural, in sentence examples

1. The classic shapes, the very full big radiuses are anatomical.

2. This enhances the driver’s capability to operate in close quarters or in tight turning radiuses.

3. However, the minimum turning radiuses of these products are different, making them ideal for different situations.

4. Intersection parts with small radiuses and positive guidance guarantee an easy and defined movement of the elements.

5. All units have wheelchair accessible turning radiuses in all rooms.

Origin of the word radius

From etymonline on radius:

“Cross-shaft, straight rod or bar,” from Latin radius “staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle”.

What’re personal pronouns?

What’s the difference between they’re, their, and there?

Whose vs who’s?


  1. “Radius.” Dictionary, Merriam-Webster, Accessed 14 Jan. 2023.
  2. Harper Douglas, “Etymology of radius,” Online Etymology Dictionary, accessed January 15, 2023,

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