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Maths Exercices

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 

= 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.

π ≈ 3.14

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.