Practice Circumference and area of a circle
A circle of radius r has circumference 2πr and area πr².
Distinguishing radius from diameter prevents common errors when measuring round objects.
Exercises with solutions
For every circle the circumference is a little more than three times the diameter. Calculations use π ≈ 3.14. The circumference is U = 2 · π · r or U = π · d, and the area is A = π · r².
Exercise 1
A circle has radius 5 cm. Calculate the circumference.
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about 31.4 cm
U = 2 · π · r = 2 · 3.14 · 5 cm = 31.4 cm.
Exercise 2
A circle has diameter 10 cm. Calculate the area.
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about 78.5 cm²
The radius is half the diameter, so 5 cm. A = π · r² = 3.14 · 25 cm² = 78.5 cm².
Exercise 3
Calculate the circumference of a circle with radius 2 m.
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about 12.56 m
U = 2 · 3.14 · 2 m = 12.56 m.
Exercise 4
How do you tell whether the circumference or the area is asked for?
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from the unit
The circumference is a length with the unit cm or m. The area is a surface and always has a square unit such as cm² or m².
Common mistake
Confusing radius and diameter. In A = π · r² you need the radius, not the diameter. Also watch the unit: areas always carry a square unit.
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