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Radius vs. Radii: What's the Difference?

Edited by Aimie Carlson || By Janet White || Updated on October 12, 2023
"Radius" is the distance from the center of a circle to its edge, while "Radii" is the plural form of "Radius," indicating multiple such distances.

Key Differences

Radius is a term that originates from geometry. It signifies the distance from the center of a circle to any point on its circumference. Thus, when you speak about the radius of a particular circle, you're referring to a specific, singular measurement that defines half of that circle's width. On the other hand, Radii is the pluralized version of this term. When one mentions multiple circles or several measurements from a center to the periphery, "Radii" is the appropriate term to use.
Additionally, the concept of Radius isn't confined to just the domain of mathematics. It can also denote areas of influence, reach, or specific regions within a given distance from a central point. Conversely, Radii can be used in contexts where multiple such areas or regions are being discussed, encompassing various distances or spheres of influence.
When it comes to linguistic usage, Radius is singular, and you would use it when discussing a single distance or aspect. Radii, being plural, implies a discussion involving multiple distances or attributes. It's essential to differentiate between these terms to ensure clarity in communication, especially in scientific or mathematical contexts.
The application of the terms Radius and Radii goes beyond circles. They can also refer to bones in the forearm (with Radius being one of the main bones), and again, "Radius" would denote one such bone, while "Radii" would signify more than one. Both words, though closely related, have distinct roles in English, emphasizing the number of distances or elements being referenced.

Comparison Chart



Refers to

One distance from the center to the edge
Multiple such distances

Usage in Geometry

Describes a singular circle's attribute
Describes attributes of multiple circles

Linguistic Role

Single term
Pluralized version

Anatomical Reference

One of the main bones in the forearm
More than one such bone

Radius and Radii Definitions


A straight line from the center of a circle to its edge.
The radius of this circle is 5 cm.


Multiple extents or reaches from various centers.
Different radio stations have different broadcasting radii.


One of the two main bones in the forearm.
He fractured his radius during the fall.


Multiple areas or distances from central points.
The company has several service radii.


The extent or reach of something from a center.
The radio has a radius of 20 miles.


Plural of radius, indicating multiple distances from centers to edges.
The radii of these circles are all different.


A specific area or distance from a central point.
The explosion affected everything within a 10-mile radius.


Several lines or segments from the centers of circles to their peripheries.
Draw radii from the center to the marked points.


A line segment that joins the center of a circle with any point on its circumference.


Several defining features or characteristics of circles.
The radii in this diagram are color-coded.


A line segment that joins the center of a sphere with any point on its surface.


A plural of radius.


A line segment that joins the center of a regular polygon with any of its vertices.


The length of any such line segment.


A circular area measured by a given radius
Every family within a radius of 25 miles of the city center.


A bounded range of effective activity or influence
The operating radius of a helicopter.


A radial part or structure, such as a mechanically pivoted arm or the spoke of a wheel.


A long, prismatic, slightly curved bone in humans, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.


A similar bone in many other vertebrates.


(anatomy) The long bone in the forearm, on the side of the thumb.


(zoology) The lighter bone (or fused portion of bone) in the forelimb of an animal.


(entomology) One of the major veins of the insect wing, between the subcosta and the media; the vein running along the costal edge of the discal cell.


(geometry) A line segment between any point of a circle or sphere and its center.
Fatima claims to have visited all the bars within a five-mile radius of her Manhattan apartment.


(geometry) The length of this line segment.


Anything resembling a radius, such as the spoke of a wheel, the movable arm of a sextant, or one of the radiating lines of a spider's web.


A right line drawn or extending from the center of a circle to the periphery; the semidiameter of a circle or sphere.


The preaxial bone of the forearm, or brachium, corresponding to the tibia of the hind limb. See Illust. of Artiodactyla.


A ray, or outer floret, of the capitulum of such plants as the sunflower and the daisy. See Ray, 2.


The barbs of a perfect feather.


The movable limb of a sextant or other angular instrument.


The length of a line segment between the center and circumference of a circle or sphere


A straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere)


A circular region whose area is indicated by the length of its radius;
They located it within a radius of 2 miles


The outer and slightly shorter of the two bones of the human forearm


Support consisting of a radial member of a wheel joining the hub to the rim


A characteristic or defining feature of a particular circle.
Each circle has a unique radius.


How do I measure the radius of a circle?

Measure the distance from the circle's center to its edge.

Are radii and radius the same thing?

"Radii" is the plural of "radius."

Is radius just for circles?

Primarily, but it can also denote areas of influence or the bone in the forearm.

How is the radius related to a circle's diameter?

The radius is half the diameter.

What is a radius in geometry?

The distance from the center of a circle to its edge.

When do we use "radii" instead of "radius"?

Use "radii" when referring to multiple distances or circles.

How does the radius affect the area of a circle?

The area is π times the radius squared.

Can an oval have a radius?

major and minor.

How are "radii" used in real-world applications?

In city planning, broadcasting, and more to denote areas of influence.

Is there a maximum limit to a circle's radius?

In theoretical terms, no; in practical applications, it depends on context.

How do "radius" and "diameter" relate to "circumference"?

Circumference is 2π times the radius or π times the diameter.

Why do some shapes have variable radii?

Shapes like ellipses have major and minor radii due to their elongated nature.

Can a circle have multiple radii?

Yes, any line from the center to the edge is a radius.

Can "radius" be used metaphorically?

Yes, as in "he has a large radius of influence."

Why is "radii" the plural form and not "radiuses"?

It follows Latin pluralization conventions.

Can "radii" refer to bones as well?

Yes, when speaking of multiple radius bones.

Is the radius always constant in a circle?

Yes, all radii in a given circle are equal.

Why is understanding "radius" important in geometry?

It's fundamental for calculations involving circles.

In which subjects apart from math might I encounter "radius" and "radii"?

Physics, engineering, anatomy, and geography, among others.

What does "within a 5-mile radius" mean?

It means within 5 miles from a specific central point.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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