circles: radius, diameter and π
Measure the diameter and radius of a circle. Wrap a tape round a circle and find how many diameters make the circumference.
📖 Start here: read the steps of the “Lesson” in order (1, 2, 3 …) and answer each question. Then continue with “Practice”.
- UK: Year 6
- France: grade 4
- Germany: grade 8
- Iran: grade 5
- Afghanistan: grade 4
- Syria: grade 5
- Palestine: grade 6
- Ukraine: grade 6
- Russia: grade 4
Guide
A diameter is a straight line through the centre of a circle, from one side to the other.
A radius goes from the centre to the circle. A diameter is two radii: diameter = 2 × radius.
The circumference is the distance all the way round a circle. Wrap a string or tape round it, unroll it and measure it with a ruler.
The circumference of every circle is about 3.14 times its diameter. This number is called pi, written π.
circumference = π × diameter ≈ 3.14 × diameter
Drag the ruler so its 0 is at the start of what you measure.