Plural of Radius
“Radius” has two accepted plural forms: “radii” and “radiuses.” Cambridge gives “radii” as the plural, while Merriam-Webster records “radii” and also “radiuses.” Use either only where your style or field permits it; do not invent a universal technical-versus-general rule that the dictionaries themselves do not establish.
Both “radii” and “radiuses” are dictionary-recorded plurals of “radius”; “radii” is the form foregrounded by Cambridge.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| one radius | radius | Singular. |
| more than one | radii | Accepted plural; foregrounded by Cambridge. |
| more than one | radiuses | Also accepted by Merriam-Webster. |
Quick tests
Plural-choice test
Are you referring to more than one radius?
If yes: Use “radii” or accepted “radiuses,” subject to your style guide.
If no: Use singular “radius.”
Caveat: A specialist publication may prefer one form, but that is a style decision rather than a universal English rule established here.
Examples
The two circles have different radii.
“Radii” is an accepted plural.
The software compares several radiuses automatically.
“Radiuses” is also dictionary-recorded.
Measure the radius of the inner circle.
One takes singular “radius.”
Further guidance
Core difference
Use “radii” or “radiuses” for more than one radius; “radii” is the more prominently listed form in the reviewed sources.
Fast Test
If a house or specialist style guide controls the document, follow it; otherwise both forms are dictionary-recorded.
Usage Boundary
Do not label “radiuses” wrong or claim a universal geometry/anatomy split without stronger specialist evidence.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- RADIUS Definition & Meaning - Merriam-Webster (opens in a new tab)Merriam-Webster · Merriam-Webster records plural radii and also radiuses across the ordinary geometry, anatomy, and related senses.
- RADIUS | English meaning - Cambridge Dictionary (opens in a new tab)Cambridge University Press · Cambridge foregrounds radii as the plural of radius in geometry, distance, and anatomy treatment.