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← Back to Geometry
1Lines, Rays, Segments2Types of Angles3Triangles by Sides4Triangles by Angles5Quadrilaterals63D Solids7Parts of a Circle8Line Symmetry9Rotational Symmetry

Concept 7 of 9

Parts of a Circle: Centre, Radius, Diameter, Chord

1

Today's big idea

Radius runs centre to edge, and a chord joins any two points on the curve.

Diameter is the one chord through the centre, which makes it the longest.

2

How it works

👀 What's going on?

Draw round a bangle and a circle appears. Every point on that curve sits the same distance from one middle spot. That single fact gives the circle all of its parts.

💡 Here's the trick

Parts of a CircleORadiusDiameter (= 2×r)ChordOuter curve = circumference · O = centre

Look at the diagram above. The middle spot is the centre, and the distance out to the curve is the radius. A chord is a straight line joining any two points on the curve. A chord that passes through the centre is the diameter. The whole curve round the outside is the circumference.

🪄 How to do it

  1. 1.Trace where each end of the line sits in the diagram above.
  2. 2.Centre to curve is a radius, curve to curve is a chord.
  3. 3.A chord through the centre is a diameter, worth two radii.

✍️ Try it

A straight line joins two points on the edge of a circle and misses the middle. What is it called? Then: a round table measures 90 cm from its middle out to the edge. How wide is the table straight across?

A straight line joining two points on the curve is a chord. The table is 90 cm from middle to edge, so straight across it is 2 × 90 = 180 cm.

⚠️ Watch out

You might be thinking that a diameter and a chord are two separate things. A diameter is simply the one chord that passes through the centre. So every diameter is a chord, and no other chord can be longer.

🌟 Why this matters

Wheels, plates, clock faces and manhole covers are all circles, and shops sell them by width across. A cyclist who buys a 26 inch wheel is choosing the distance right across it. Knowing the parts lets you turn one measurement into any other.
Exam tip: Underline whether the question hands you a radius or a diameter before working anything out.

3

Watch me solve one

Quanta solves it

“Watch how I think through one of these — then you try.”

THE PROBLEM

“A circle has a radius of 9 cm. What is its diameter?”

  1. 1

    Spot the relationship

    Diameter = 2 × radius. ⭕

  2. 2

    Substitute

    d = 2 × 9

  3. 3

    Multiply

    18 cm

  4. 4

    Answer ✓

    Diameter = 18 cm.

Now you try one — same steps, different story 👇

4

See it in action

⚡ Time to play!

Tap the part named below directly on the circle. A green glow means you found it.

CIRCLE · ROUND 1 OF 4

OAPQMN

Tap the CIRCUMFERENCE (the circle's boundary).

5

Quick check

✅ VERIFICATION

What is the name for the complete distance right round the outside of a circle?

⚡ Finish the activity above first to answer.

Keep going!

Up next: Line Symmetry

Square: 4 · Rectangle: 2 · Circle: ∞ · Scalene Δ: 0.

Concept 8 of 9 →