The radius is the distance from the center to the edge. The diameter goes right across through the center, so it is twice the radius. The circumference is the distance around the circle.
For every circle, circumference ÷ diameter = π (pi), which is about 3.14. So circumference = π × diameter, and area = π × radius². A circle with radius 7 cm has circumference 3.14 × 14 = 43.96 cm and area 3.14 × 49 = 153.86 cm².
Worked example
Find the circumference of a circle with diameter 17 cm. Use π = 3.14.
- Circumference = π × diameter.
- 3.14 × 17 = 53.38.
Answer:53.38 cm
Key words
- radius
- center to edge
- diameter
- edge to edge through the center
- π (pi)
- about 3.14, circumference ÷ diameter
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Common Core-referenced: 7.G.B.4