The Circle: Centre, Radius and Diameter

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Draw a Circle in Your Copy
5 Class Challenge
6 What Did We Notice?
7 What's Next
Pupil practice

1 - Getting Started ~4 mins

Illustration for Getting Started Look at these three round things: a clock face, a bicycle wheel and a coin. Where is the very middle of each circle? And if you wanted to measure straight across the widest part, where would you put your ruler?

Teacher Notes

Show the three round objects (photo or real). Give five seconds of quiet think-time before hands go up. Take two or three answers on where the middle is and one on where to measure across. Do not name centre, radius or diameter yet — that lands in Watch and Notice.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~8 mins

Watch the circle on the board. First we see the centre — the exact middle. Then a straight line from the centre out to the edge: that line is the radius. When a second radius points the opposite way, the two together make one straight line right across the circle through the middle: that is the diameter. When we have more than one radius, we call them radii: so one radius, two radii. Notice how far the diameter reaches compared to a single radius.

Teacher Notes

Point to the centre first. Then trace the radius out to the edge. Then draw a second radius the opposite way so the class sees the two line up into one straight line across the middle — the diameter. Build each part live on the tool as you name it. Hold out for the noticing: how many radii fit into the diameter? Let a pupil say it before you confirm. Say the diameter always passes through the centre. Do not narrate the answer for them — flag what to watch and let them see it.

No IWB? Draw the circle on a plain whiteboard with a compass or a circle template, then mark the centre dot, one radius and the diameter by hand in the same order.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~11 mins

Today we fold a paper circle to find its parts. Each of you has a circle. Follow these three steps:

Fold your circle in half so the two edges land exactly on top of each other. Open it: that crease runs right across through the middle, so it is a diameter.
Fold again a different way and open it. Where the two creases cross is the centre.
Point to a radius: from that crossing point straight out to the edge.

Hands-on Task

Teacher Notes

Talk this one through together — pupils take turns at the board and the class agrees or corrects out loud.

Circulate as pupils fold. Watch for folds where the edges do not quite meet — that crease will not be a true diameter. Ask a pupil to hold up their circle and point to the centre, then a radius, then the diameter. Revoice: the crease goes through the middle, so it must be a diameter.

+15 XP

4 - Draw a Circle in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, draw a circle. Mark and label its centre. Draw and label one radius. Then draw and label its diameter. Check that your diameter line passes right through the centre.

Teacher Notes

Walk the room glancing for the diameter passing through the centre and the labels in place — this is whole-class copybook practice, not marking. A common slip is a diameter drawn near the middle but not through it; nudge those pupils to re-draw through the centre dot.

+10 XP

5 - Class Challenge ~8 mins

We take turns at the board to make these circles. Before each one, the whole class says the missing measurement out loud. The board tool is set by its radius, so for a diameter target we first halve it to find the radius to set. Say the missing measurement for each:

radius 4 cm
diameter 10 cm — what radius?
radius 3 cm
diameter 12 cm — what radius?

Match the circle

Teacher Notes

This is the practice round — pupils take turns at the board, check each answer, and the class confirms before moving on. The tool is set by radius, so a diameter target must first be halved to find the radius to set on the slider.

The diameter-target ones (10 cm and 12 cm) are where pupils think hardest — slow down on those two and give each a real predict-then-check beat. For a diameter of 10 cm the radius must be 5 cm; for 12 cm it must be 6 cm. Ask the class to predict the radius before the pupil at the board sets the slider, then press Check. Revoice the halving and doubling: the diameter is two radii, so half the diameter gives the radius.

No IWB? Run the same round with a compass on a plain board: a pupil sets the compass to the named radius, draws the circle, and the class checks the measurement with a ruler.

+15 XP

Pupil practice

Module 7 · 2D and 3D Shapes and Angles Data & Chance

Lesson 81 · The Circle: Centre, Radius and Diameter

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Copyright Notice
This lesson is copyright of Coding Ireland 2017 - 2025. Unauthorised use, copying or distribution is not allowed.

Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~8 mins

3 - Try It Together ~11 mins

Hands-on Task

4 - Draw a Circle in Your Copy ~3 mins

5 - Class Challenge ~8 mins

Match the circle

Unlock the full learning experience

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