Plural of Radius: Definition, Rules, and Usage Explained

The word radius often leaves people puzzled when it comes to its plural form. Many learners assume it follows the common rule of adding an “s” to form radiuses, while others insist the only correct form is radii. Both forms actually exist, but they serve different purposes in English. Understanding the history, grammar, and correct contexts for each plural is key to using them with confidence.

At its heart, a radius is the distance from the center of a circle to its edge, but it also carries meaning in anatomy and even in everyday speech. Because of its Latin origin, the word has two accepted plurals, and this is where confusion begins. Some prefer to use radii in technical or academic contexts, while radiuses have gained ground in modern English where simplicity dominates.

Geometry Terms

Word	Meaning	Example
Radius	Line from circle center to edge	The radius of the circle is 5 cm.
Radii	Classical plural of radius	The radii of these circles are different.
Radiuses	Modern plural of radius	The machine has several radiuses.
Diameter	Line across circle through center	The diameter equals twice the radius.
Circumference	Distance around a circle	The circumference was measured at 31.4 cm.
Chord	Line between two circle points	A chord divided the circle neatly.
Arc	Curved part of circumference	The arc covered 90 degrees.
Sector	Circle part shaped like a slice	The pizza slice looked like a sector.
Tangent	Line touching a circle once	The tangent touched the circle at point P.
Segment	Area cut off by a chord	The small segment formed a curved triangle.

Latin-Based Plurals

Word	Meaning	Example
Axis / Axes	Line of reference	The x and y axes meet at zero.
Focus / Foci	Central points	The foci of the ellipse define its shape.
Syllabus / Syllabi	Course outline	The teacher handed out syllabi.
Nucleus / Nuclei	Atom’s center	The nuclei contained protons and neutrons.
Stimulus / Stimuli	Cause of reaction	The stimuli affected the plants.
Medium / Media	Means of communication	Social media shapes our world.
Formula / Formulae	Mathematical expressions	Scientists used old formulae in calculations.
Datum / Data	Information unit	Data shows the rainfall trend.
Thesis / Theses	Research papers	Students defended their theses.
Appendix / Appendices	Extra book material	The appendices had charts and graphs.

Regular English Plurals

Word	Meaning	Example
Book / Books	Something to read	Books were stacked on the shelf.
Car / Cars	Vehicles	Cars filled the street.
House / Houses	Homes to live in	New houses are being built.
Dog / Dogs	Common pets	Dogs barked loudly outside.
Tree / Trees	Tall plants	Trees lined the road.
Chair / Chairs	Furniture for sitting	The chairs were placed neatly.
Table / Tables	Flat furniture	Tables were set for dinner.
Pen / Pens	Writing tool	Pens rolled off the desk.
Cup / Cups	Drinking vessel	Cups were stacked in rows.
Shoe / Shoes	Footwear	His shoes were muddy.

Anatomy Words

Word	Meaning	Example
Radius (Bone)	Forearm bone	The radius bone helps wrist movement.
Ulna	Bone near radius	The ulna runs beside the radius.
Femur	Thigh bone	The femur supports the body’s weight.
Tibia	Shinbone	The tibia was fractured.
Fibula	Small leg bone	The fibula gives balance.
Scapula	Shoulder blade	The scapula links the arm and body.
Clavicle	Collarbone	The clavicle supports the shoulder.
Humerus	Upper arm bone	The humerus joins the elbow and shoulder.
Vertebra / Vertebrae	Backbone bones	Vertebrae protect the spinal cord.
Phalanx / Phalanges	Finger bones	Phalanges help grip objects.

Geometry Extensions

Word	Meaning	Example
Diameter Line	Longest line across circle	The diameter line is twice the radius.
Radius Vector	Radius as a vector	The radius vector changes direction.
Polar Radius	Radius in polar coordinates	Polar radii mark different points.
Mean Radius	Average radius value	The mean radius of Earth is known.
Equatorial Radius	Radius at equator	The Earth’s equatorial radius is larger.
Polar Radius	Radius at poles	The polar radius is shorter.
Atomic Radius	Size of atom	The atomic radius decreases across a period.
Covalent Radius	Half distance between nuclei	Covalent radius defines bonding.
Ionic Radius	Ion size	The ionic radius varies with charge.
Van der Waals Radius	Non-bonded atom radius	Van der Waals radius affects interactions.

.Physics Terms

Word	Meaning	Example
Velocity	Speed in a direction	The velocity of the car was high.
Acceleration	Rate of change of velocity	The ball showed constant acceleration.
Force	Push or pull	A strong force moved the object.
Energy	Capacity to do work	Solar energy powers homes.
Mass	Amount of matter	The mass of the stone was large.
Volume	Space an object occupies	The volume of water filled the jug.
Density	Mass per volume	The density of iron is high.
Power	Rate of doing work	The motor uses a lot of power.
Pressure	Force per area	The pressure inside the tire increased.
Work	Force times distance	Work is done when a box is pushed.

Chemistry Terms

Word	Meaning	Example
Molecule	Smallest unit of a compound	A water molecule has two hydrogen atoms.
Atom	Basic unit of matter	Every atom has electrons and protons.
Ion	Charged particle	Sodium forms a positive ion.
Compound	Two or more elements	Salt is a compound of sodium and chlorine.
Element	Pure chemical substance	Oxygen is an element.
Proton	Positively charged particle	Protons are in the nucleus.
Neutron	Neutral particle	Neutrons balance protons in atoms.
Electron	Negatively charged particle	Electrons orbit the nucleus.
Bond	Link between atoms	A covalent bond shares electrons.
Reaction	Change in substances	The reaction released heat.

Astronomy Terms

Word	Meaning	Example
Planet	Body orbiting a star	Earth is a planet.
Star	Ball of gas shining by fusion	The star was bright at night.
Galaxy	System of stars and planets	The Milky Way is a galaxy.
Orbit	Path of an object	The moon’s orbit surrounds Earth.
Comet	Ice and dust object	The comet had a bright tail.
Asteroid	Small rocky body	An asteroid orbits between Mars and Jupiter.
Meteor	Rock entering atmosphere	The meteor glowed brightly.
Satellite	Object orbiting a planet	The satellite sent images of Earth.
Eclipse	One body blocks another	A solar eclipse darkened the sky.
Universe	Everything in space	The universe is vast.

Environmental Science

Word	Meaning	Example
Climate	Weather pattern over time	Climate is warming globally.
Ecosystem	Living and nonliving system	The forest ecosystem supports animals.
Pollution	Harmful substances in environment	Pollution damages rivers.
Recycling	Reusing materials	Recycling saves energy.
Habitat	Natural home of organisms	The desert is a harsh habitat.
Deforestation	Cutting down forests	Deforestation reduces biodiversity.
Biodiversity	Variety of life	Biodiversity is important for balance.
Conservation	Protecting resources	Conservation protects endangered species.
Renewable	Resource replaced naturally	Solar power is renewable.
Sustainability	Meeting needs without harm	Sustainability guides future planning.

Mathematics Terms

Word	Meaning	Example
Equation	Statement of equality	The equation was solved easily.
Function	Relation of inputs and outputs	The function increased steadily.
Variable	Symbol for unknown	x is a variable.
Constant	Fixed value	Pi is a constant.
Angle	Space between lines	The angle measured 60 degrees.
Triangle	Shape with three sides	A triangle has 180 degrees.
Square	Shape with four equal sides	The square had equal angles.
Circle	Shape with all points equal distance	The circle had radius 5 cm.
Polygon	Shape with many sides	The polygon had eight sides.
Theorem	Proven mathematical statement	Pythagoras’ theorem relates sides of a triangle.

Technology Terms

Word	Meaning	Example
Computer	Machine for processing	The computer runs fast.
Software	Programs for computers	The software updated quickly.
Hardware	Physical computer parts	Hardware includes the keyboard.
Internet	Global network	The internet connects people.
Database	Organized data collection	The database stored records.
Algorithm	Step-by-step process	The algorithm solved the problem.
Code	Programming instructions	The code ran smoothly.
Robot	Machine that performs tasks	The robot cleaned the floor.
Network	Linked systems	The network connects offices.
Server	Computer providing services	The server handled many users.

Geology Terms

Word	Meaning	Example
Rock	Solid mineral mass	The rock was heavy.
Mineral	Natural solid substance	Quartz is a mineral.
Soil	Upper earth layer	Soil grows plants.
Volcano	Mountain that erupts lava	The volcano erupted suddenly.
Earthquake	Shaking of ground	The earthquake damaged houses.
Fossil	Remains of ancient life	The fossil was millions of years old.
Crystal	Solid with ordered structure	The crystal shone brightly.
Magma	Molten rock inside Earth	Magma rises to the surface.
Lava	Molten rock outside Earth	Lava flowed down the mountain.
Plate	Large earth slab	Plates move slowly.

Grammar and Language

Word	Meaning	Example
Noun	Name of thing	“Dog” is a noun.
Verb	Action word	“Run” is a verb.
Adjective	Describes noun	“Happy” is an adjective.
Adverb	Describes verb	“Quickly” is an adverb.
Pronoun	Replaces noun	“He” is a pronoun.
Plural	More than one	Cats is plural.
Singular	One only	Cat is singular.
Synonym	Similar meaning	Big is a synonym for large.
Antonym	Opposite meaning	Hot is an antonym of cold.
Sentence	Group of words	The sentence was simple.

Mixed Science

Word	Meaning	Example
Gravity	Force pulling to Earth	Gravity keeps us grounded.
Light	Visible energy	Light filled the room.
Sound	Vibration heard by ears	Sound echoed in the cave.
Heat	Energy of motion	Heat warmed the air.
Wave	Repeating disturbance	The wave moved through water.
Current	Flow of electricity	The current shocked him.
Circuit	Path for electricity	The circuit was broken.
Magnet	Object that attracts iron	The magnet stuck to the fridge.
Charge	Property of particles	Electrons carry negative charge.
Field	Space influenced by force	The magnetic field was strong.

Conclusion

The plural of radius comes in two forms: radii and radiuses. Both are correct, and both have a place in English. The form you choose depends on your purpose, your readers, and your tone. Academic and scientific contexts favor radii, while casual or everyday writing accepts radiuses without issue.

By mastering these plurals, you not only avoid mistakes but also show your flexibility with English. Whether you use radii to discuss geometry or radiuses in casual conversation, you now know the difference. Understanding this balance makes your writing stronger and your communication clearer.