Pi and Circles
Pi (pi) is the ratio of a circle's circumference to its diameter, approximately 3.14159. It is used to calculate the circumference and area of circles and appears throughout mathematics and science.
What You Need to Know
Key Concept Diagram
Pi is the ratio C/d for any circle, approximately 3.14159 (or 22/7 as an approximation)
Circumference = pi x diameter = 2 x pi x radius
Area of a circle = pi x radius squared (A = pi r^2)
Pi is irrational: its decimal expansion never ends or repeats
The radius is half the diameter; r = d/2
Key Vocabulary
Pi
The ratio of a circle's circumference to its diameter, approximately 3.14159, symbol pi
Circumference
The perimeter (total distance around) a circle; C = pi x d
Radius
The distance from the centre of a circle to its edge; half the diameter
Diameter
The distance across a circle through its centre; d = 2r
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
A circle has a diameter of 10 cm. What is its circumference? (Use pi = 3.14)
Question 2
What is the area of a circle with radius 5 m? (Use pi = 3.14)
Question 3
Which statement best describes pi?
Key Concepts Summary
- ●Pi is the ratio C/d for any circle, approximately 3.14159 (or 22/7 as an approximation)
- ●Circumference = pi x diameter = 2 x pi x radius
- ●Area of a circle = pi x radius squared (A = pi r^2)
- ●Pi is irrational: its decimal expansion never ends or repeats
- ●The radius is half the diameter; r = d/2