The Plural of Radius: Here’s What It Is and How to Use It

If you have ever second-guessed yourself on what the plural form of radius was and also wanted to learn more about the word, it’s grammar, it’s history, and plenty of examples of it used in a sentence, this article is practically made for you! 

Your writing, at its best

Compose bold, clear, mistake-free, writing with Grammarly's AI-powered writing assistant

What Is the Definition of Radius?

According to, the word radius has these definitions and uses. 

  • a line segment extending from the center of a circle or sphere to the circumference or bounding surface
    •  the bone on the thumb side of the human forearm next to the ulna
  • also: a corresponding part of vertebrates above fishes
    • the third and usually largest vein of an insect’s forelimb
  • the length of a radius
    • a truck with a short turning radius
  • the circular area defined by a stated radius
    • a bounded or circumscribed area
  • a radial part
    • the distance from a center line or central point to an axis of rotation
    • the length of this line is called the radius

What Is the Difference Between Radius and Radi?

Radius is the singular form and one of the plural forms of the word radius is radii. So if you are referring to the radius of two objects or more then you could say “the radii of the two objects”. 

There is also the plural form more common for conventional English speakers radiuses. Depending on whether or not you are using it in a formal sense, like a paper or homework, you can use either plural form. The more accepted plural form of the word is radiuses, based on the Latin plural. 

The History and Origin of the Word

The word radius has an origin dating back to 1578. It was used in the context of the first definition listed above “a line segment extending from the center of a circle or sphere to the circumference or bounding surface”. 

The word radius can be traced back to the Latin language. It is a word borrowed from the Latin radius and used in the same spelling and context today in American English. The definition in Latin is staff, spoke, ray, spoke of a wheel, ray of light, beam of light, radius of a circle. First used for geometry in the 1610s. 

Example Sentences of the Word in Context

Synonyms of Radius from the Thesaurus

  • Area – region or part of a town, country, or the world
  • Bounds – a figurative limit
  • Circle – something in the shape of a circle
  • Range – maximum distance of capability
  • Remit – to transmit, supply or send, especially money or payment
  • Scope – the extent of the area or subject matter that something deals with or to which it is relevant
  • Sweep – to brush off or brush away, typically dirt or litter, to art or turn away from a straight line or course
  • Umbrella – a device consisting of a circular canopy of cloth on a folding metal frame supported by a central rod, used as protection against rain
  • Ambit – the action or process of enlarging or extending something
  • Area of influence – range, sweep, area or sphere of activity regarded as someone’s personal territory 
  • Boundary – outside limit of an object, area or surface
  • Compass – range or scope of something 
  • Expanse – a wide stretch, usually of sea, sky, or land
  • Spoke – spoke of a tire


Now you are no doubt an expert on using the correct plural form of the word radius in the English language and hopefully also took away a lot more about the word. Next time you are writing a paper or doing your homework you can confidently use the correct plural form.