Year 8 · Maths · Area of trapezia and circles · Lesson 2
Circumference of a circle
Name the parts of a circle and find the circumference using C = πd and C = 2πr, including answers in terms of π and problems.
27-minute video lesson
Teaching, worked examples and 20 practice questions solved step by step.
Watch free for a monthWhat your child will learn
- Name the parts of a circle
- Understand what π is
- Find the circumference from the diameter
- Find the circumference from the radius
- Solve problems involving circumference
The method
- If you know the diameter: C = π × d.
- If you know the radius: C = 2 × π × r.
- Use the π button, then round at the end.
- 'In terms of π': leave π in the answer.
Key words: circle, radius, diameter, circumference, pi.
Worked examples
The first three practice questions from the lesson, with every step.
Question 1
Find the circumference of a circle with diameter 6 cm. Give your answer to 1 decimal place.
- Step 1: C = π × d
- Step 2: C = π × 6 = 18.8496...
- Step 3: C = 18.8 cm (1 d.p.)
Answer: 18.8 cm
Question 2
Find the circumference of a circle with diameter 15 cm. Give your answer to 1 decimal place.
- Step 1: C = π × d
- Step 2: C = π × 15 = 47.1239...
- Step 3: C = 47.1 cm (1 d.p.)
Answer: 47.1 cm
Question 3
Find the circumference of a circle with diameter 9 m. Give your answer to 1 decimal place.
- Step 1: C = π × d
- Step 2: C = π × 9 = 28.2743...
- Step 3: C = 28.3 m (1 d.p.)
Answer: 28.3 m