Parts of a circle
A circle has a centre, a radius, a diameter and a circumference, plus chords, arcs and sectors. Labelled pictures and a quiz below.
🧒 Kids: radius or diameter?
The radius goes from the middle to the edge. The diameter goes all the way across, through the middle. It is twice as long!
▶ Quiz 🔷 More practice 📐 Area and perimeter
💡 Trick: Diameter = 2 × radius. Radius = diameter ÷ 2.
More on shapes and lines: Lines, rays and segments · Polygons · Types of triangles · Quadrilaterals · Parts of a circle · Faces, edges and vertices
This part is for grown-ups: the key idea, pictures to draw, what to say, common mistakes, questions with answers and free printables.
The trick: diameter is double
Ask your child to trace the radius with a finger, from the middle to the edge. Then trace the diameter: middle to edge, and the same again on the other side. Two radii make one diameter.
- Radius 4 cm: diameter 8 cm
- Diameter 10 cm: radius 5 cm
- Radius 2.5 m: diameter 5 m
- Diameter 7 in: radius 3.5 in
Patterns to spot
- Every radius of a circle is the same length.
- A diameter is the longest chord.
- Every diameter passes through the centre.
- A circle has as many lines of symmetry as you like: every diameter is one.
- Half a circle is a semicircle; a quarter is a quadrant.
- Circumference ÷ diameter is always about 3.14 (π).
Key words
- Centre: the point in the middle, the same distance from every point on the circle.
- Radius: a line from the centre to the circle. Plural: radii.
- Diameter: a line across the circle through the centre. It is 2 radii long.
- Circumference: the distance all the way round the circle.
- Chord: a line joining two points on the circle. The diameter is the longest chord.
- Arc and sector: an arc is part of the circumference; a sector is a slice, like a pizza slice.
Why it matters: Every circle formula uses the radius or the diameter. Knowing the parts makes circumference and area easy to learn later.
See it: pictures to draw with your child
Draw these on paper, or build them with straws, paper and boxes. Holding and turning real shapes makes the words stick.
What your child already knows
Your child already knows a circle is round and has no corners. Now they learn the names for the lines inside it, which they will use for every circle formula.
In real life: Bike wheels have spokes like radii, and a pizza cut into slices shows sectors. Measure across a plate through the middle: that is its diameter.
Radius and diameter
The diameter is always twice the radius, and the radius is always half the diameter.
| Radius | Diameter |
|---|---|
| 1 cm | 2 cm |
| 3 cm | 6 cm |
| 5 cm | 10 cm |
| 8 cm | 16 cm |
| 12 cm | 24 cm |
So diameter = 2 × radius, and radius = diameter ÷ 2. Check which one a question gives before using a formula: it is the most common slip.
The circumference is a little more than 3 diameters. That number, 3.14…, is called pi (π). See the calculator to try it.
Worked examples
Example 1: Find the diameter
- The radius is 6 cm.
- The diameter is 2 × 6 = 12 cm.
Example 2: Find the radius
- The diameter is 18 m.
- The radius is 18 ÷ 2 = 9 m.
Example 3: Name the part
- A line joins two points on the circle but misses the centre.
- It is a chord, not a diameter.
Common mistakes
What to say to your child
A short script for your first session. Pause after each line and let your child answer.
- “Put your finger on the centre.”
- “Trace from the centre to the edge. What is that line?” (A radius.)
- “Now go all the way across through the centre.” (A diameter.)
- “How many radii make a diameter?” (2.)
- “What do we call the distance round the edge?” (The circumference.)
- “What is a slice called?” (A sector.)
Questions to try (answers below)
Ask these out loud, with pictures or real objects to hand. The last one is a stretch.
- Radius 5 cm. Diameter?
- Diameter 14 m. Radius?
- Which is the longest chord?
- What is the distance round a circle called?
- A pizza slice is a…
- Are all the radii of a circle the same length?
- Radius 1.5 in. Diameter?
- Part of the circumference is called an…
- Stretch: a circle fits exactly inside a 20 cm square. What is its radius?
Show the answers
- 10 cm
- 7 m
- the diameter
- the circumference
- sector
- Yes
- 3 in
- arc
- 10 cm
Games to play at home
String circles
Tie a pencil to a string and pin the other end. Draw a circle: the string is the radius.
Plate measure
Measure across plates and lids through the middle. Halve it to find the radius.
Pizza parts
At pizza time, name the crust (circumference), a cut through the middle (diameter) and a slice (sector).
A 5-day plan for parts of a circle
Ten minutes a day is enough. Short, regular practice works far better than one long session.
- Day 1: Learn it with the pictures above: draw them on paper or build them with real objects, and talk them through.
- Day 2: Read the script together, then ask the first four questions.
- Day 3: Do the quiz at the top of the page and read the working for anything missed.
- Day 4: Play one of the games at home for ten minutes.
- Day 5: Ask the rest of the questions, then do a round of the matching practice skill.
By grade
Children meet circles in kindergarten, but the parts are named from grade 3 or 4. Common Core 7.G.B.4 uses the radius and diameter in the circumference and area formulas in grade 7.
CBSE families: Circles (class 9) and Circles (class 10) build on these parts.
More for each grade: Grade 3 math and the CBSE class 3 maths guide; Grade 4 math and the CBSE class 4 maths guide; Grade 5 math and the CBSE class 5 maths guide; Grade 6 math and the CBSE class 6 maths guide; Grade 7 math and the CBSE class 7 maths guide.
What comes next
Next, use the radius and diameter in the circumference and area formulas with the circle calculator, then see how to find area.
Worksheets and more practice
Print a free worksheet with an answer key, or keep practicing online with geometry practice.
Naming 2D shapes
Fractions of shapesMore printables: area and circumference of circles worksheet · fractions of shapes worksheet · make a fresh test
More: geometry for kids · shapes for kids · practice · 2D shapes chart · 3D shapes chart
Last reviewed
Questions parents ask
What are the parts of a circle?
The centre, radius, diameter, circumference, chord, arc and sector.
What is the difference between radius and diameter?
The radius goes from the centre to the edge. The diameter goes across through the centre and is twice as long.
How do you find the radius from the diameter?
Divide the diameter by 2. A diameter of 12 cm gives a radius of 6 cm.
What is the circumference of a circle?
The distance all the way round it. It is π × diameter, a little more than 3 diameters.
Is a diameter a chord?
Yes. It is the longest chord, because it passes through the centre.