What’s the Plural of Radius: Understanding Geometric Terms

Radius, Radii, Radiuses — See the Pattern

Look at these words. Can you see what happens?

• one radius → two radii
• one cactus → two cacti
• one focus → two foci
• one stimulus → two stimuli

Did you see the pattern? These words all come from Latin. In Latin, words ending in -us change to -i for the plural. So radius becomes radii.

You can also use the normal English rule and add -es to get radiuses. Both are correct, but radii is the one you will see most often.

When to Use Radii and When to Use Radiuses

Both words mean the same thing. But people use them in different places.

Use radii when:

• You are writing for school, math, or science → “The radii of these three circles are 4, 6, and 9 cm.”
• You want to sound more formal → “We measured the radii of several wheels.”

Use radiuses when:

• You are writing in everyday English → “The delivery app shows different radiuses on the map.”
• You want to keep things simple → “They checked the radiuses of both pipes.”

Easy way to remember: If you are not sure, use radii. It works everywhere and most people prefer it.

Real-Life Examples With Radius and Radii

• The pizza shop delivers within a 5-mile radius of the store. (one radius — a distance from the centre)
• The teacher drew two circles with different radii on the board. (more than one — the lines from centre to edge)
• My Wi-Fi only works within a small radius of the house. (one radius — a range or area)
• The engineer compared the radii of the two wheels. (more than one)
• Is there a good restaurant within a 10-minute radius? (one radius — a distance)

You are doing great. Now let’s look at the mistakes many learners make with this word.

Two Mistakes to Avoid With Radius

Even native speakers sometimes pause before saying the plural of radius — so if you get confused, you are not alone. Once you learn the pattern, it sticks.

How to remember: Think of the letter i. Radius drops the -us and adds -i — just two letters instead of two. Radi + i = radii.

Other words that follow this pattern: cactus → cacti, focus → foci, nucleus → nuclei, stimulus → stimuli, syllabus → syllabi.

Test Yourself: Radius or Radii?

Choose the correct answer for each sentence. Click Check to see if you are right.

Keep Going — You Are Building Something

You just learned that radius becomes radii (or radiuses). That is one more Latin plural you will never get wrong again.

But here is something interesting. The word focus also comes from Latin and follows the same pattern. So is the plural “focis”? Or “foci”? Or “focuses”? The answer is not as simple as you might think — because one of those options is actually wrong.

Next lesson: What Is the Plural of Focus?

Sources

Etymology of radius, Online Etymology Dictionary.