Radius is a common term used in geometry, and it refers to the distance between the center of a circle and its circumference. When it comes to the plural form of radius, many people find it confusing. Some may think the plural of radius is “radiuses,” while others may believe it is “radii.” In this article, we will explore the correct grammar rules for the plural of radius and provide examples to help you understand it better.
Understanding Radius and Plural of Radius
What Does “Radius” Mean?
The word radius has more than one meaning, depending on the context:
1. In geometry:
The radius is the distance from the center of a circle or sphere to its outer edge.
-
Example: The radius of the circle is 5 cm.
2. In anatomy:
The radius is one of the two main bones in your forearm (the other is called the ulna).
-
Example: She broke her radius while playing basketball.
3. In general use:
It can also refer to a range or distance around a central point.
-
Example: There are five restaurants within a two-mile radius of the hotel.
What Is the Plural of “Radius”?
✅ The plural of radius can be radii or radiuses, depending on the context.
| Singular | Plural (formal/scientific) | Plural (common/modern) |
|---|---|---|
| radius | radii | radiuses |
Both are grammatically correct — but they are used in different situations.
When to Use “Radii”
Radii (pronounced /ˈreɪ.di.aɪ/) is the traditional Latin plural form of radius. It is the preferred choice in mathematical, scientific, and technical contexts.
Examples:
-
The radii of these circles are all equal.
-
Scientists measured the radii of several atoms.
-
The radii of the planets vary depending on their size.
-
In geometry, two radii make a diameter.
When to Use “Radiuses”
Radiuses (pronounced /ˈreɪ.di.əs.ɪz/) is the modern English plural form. It’s more common in everyday English or non-technical contexts, where simplicity and clarity matter more than Latin accuracy.
Examples:
-
The company operates in several radiuses around the city.
-
We drew three different radiuses on the chart.
-
The circles have equal radiuses in this diagram.
✅ Both radii and radiuses are correct, but radii sounds more formal or academic, while radiuses is easier for general conversation.
Examples in Sentences
Singular: “Radius”
-
The radius of the circle is 4 cm.
-
A larger radius means a wider circle.
-
The surgeon examined the patient’s radius bone.
-
There are three hospitals within a 10-mile radius.
-
The radius of this sphere is half its diameter.
Plural: “Radii” (Formal / Scientific)
-
The radii of the planets are listed in the table.
-
In geometry, all radii of a circle are equal in length.
-
The radii of these two circles are in a 2:1 ratio.
-
Astronomers calculated the radii of distant stars.
-
Two radii form one diameter in a circle.
Plural: “Radiuses” (General Use)
-
The company delivers products within three radiuses of the city center.
-
The students measured the radiuses of several wheels.
-
We compared the radiuses of different circles on the map.
-
The radiuses vary slightly between models.
-
He drew multiple radiuses before shading the circle.
Common Mistakes English Learners Make
Mistake 1: Mixing singular and plural forms
-
Incorrect: The radii is 4 cm.
-
✅ Correct: The radii are 4 cm.
Mistake 2: Using “radiuses” in technical writing
-
Incorrect: The radiuses of atoms are measured in picometers.
-
✅ Correct: The radii of atoms are measured in picometers.
Mistake 3: Forgetting that “radius” has two plural forms
-
Incorrect: I’m not sure what the plural of radius is.
-
✅ Correct: The plural can be radii or radiuses, depending on context.
Practice Exercises
Exercise 1: Fill in the blanks with radius, radii, or radiuses.
-
The ______ of the circle is 6 inches.
-
The scientist compared the ______ of several planets.
-
The students drew different ______ to complete the diagram.
-
The map shows all towns within a 10-mile ______.
-
The ______ of both wheels are the same size.
Answer:
-
radius
-
radii
-
radiuses
-
radius
-
radii
Exercise 2: Choose the correct sentence.
-
a) The radii of all circles are equal. ✅
b) The radiuses of all circles are equal. -
a) The radius of the Earth is about 6,371 km. ✅
b) The radii of the Earth is about 6,371 km. -
a) Astronomers measured the radii of distant planets. ✅
b) Astronomers measured the radiuses of distant planets. (acceptable, less formal) -
a) The company delivers products within several radiuses of downtown. ✅
b) The company delivers products within several radii of downtown. (less natural) -
a) The radii were drawn from the circle’s center. ✅
b) The radius were drawn from the circle’s center.