Circles: calculate circumference, radius, and diameter
The diameter of a circle is 10 millimetres. What is the circle's radius? 


Use this formula to relate the radius and the diameter.
d = 2 x r
The diameter is equal to 10 millimetres. Replace d by 10 in the formula.
d 
= 2 × r 

10 
= 2 × r 

10 ÷ 2 
= r 

5 
= r 
The radius is 5 millimetres.
The circumference of a circle is the distance around the outside, just as the perimeter of a polygon is the distance around the outside. The circumference of a circle can be found by measuring its radius or diameter and the using the formula:
C = 2 × π × r (or C = π × d)
The radius of a circle is 2 centimetres. What is the circle's circumference? 


Use 3.14 for π.
Use this formula to find the circumference from the radius.
C = 2 × π × r
Use 3.14 as an approximation for π.
π ≈ 3.14
The radius is 2 centimetre. Replace r by 2 in the formula.
C 
= 
2 × π × r 


≈ 
2 × 3.14 ×2 


≈ 
12.56 
The circumference is about 12.56 centimetres.
The diameter of a circle is 8 metres. What is the circle's circumference? 


Use 3.14 for π.
Use this formula to find the circumference from the diameter.
C = π × d
Use 3.14 as an approximation for π.
The diameter is 8 metres. Replace d by 8 in the formula.
C 
= 
π × d 


≈ 
3.14 ×8 


≈ 
25.12 
The circumference is about 25.12 metres.
The circumference of a circle is 12.56 metres. What is the circle's diameter? 


Use 3.14 for π.
Use this formula to relate the diameter and the circumference.
C = π × d
Use 3.14 as an approximation for π.
π ≈ 3.14
The circumference is 12.56 metres. Replace C by 12.56 in the formula.
C 
= π × d 

12.56 
≈ 3.14 × d 

12.56 ÷ 3.14 
≈ d 

4 
≈ d 
The diameter is about 4 metres.