Straight across the middle
Lay a ruler across a glass bangle. Across the middle, the ruler shows the biggest number.
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The rule
Go from the centre to the edge of the circle. That is one radius.
Continue through the centre to the other side. That is a second radius.
The whole line is the diameter. It is twice the radius.
Example
Radius 30 mm gives diameter 60 mm. The mm means millimetres.
Double the radius: 2 × 30 = 60 mm.
Diameter 70 mm gives radius 35 mm.
Divide the diameter by 2: 70 ÷ 2 = 35 mm.
Watch out!
An edge-to-edge line is a diameter only through the centre. A line that misses the centre is shorter.