Here are two circles side by side: a bicycle wheel and a dinner plate. One is big, one is small. What is the same about both of them, and what one measurement could you take that would tell you nearly everything about the size of a circle?

Look at the circles on screen. The first shows the parts of a circle: the centre in the middle, a radius reaching out to the edge, the diameter going all the way across through the centre, and the circumference running all the way around the outside edge. Notice how the diameter is exactly twice the radius.

Now a bigger circle, same rule. This one shows the radius, say what the diameter will be before you check.

And here the diameter is given first. Say what the radius has to be when we know the whole width across.

Now the written conversion round in your copy. For each circle you are given one length and you find the other. Work them in this order, one under the other:

• radius 3 cm → find the diameter
• radius 8 cm → find the diameter
• diameter 14 cm → find the radius
• diameter 9 cm → find the radius (an odd number — watch the half)
• diameter 24 cm → find the radius