The circumference is the length of the edge around a circle.
For any circle, the circumference is:
3.141592... x the diameter.
Or in symbols: C = (3.141592...)d.
This is true for all circles and so 3.1414592... is therefore a special, unique number, and we represent it with the Greek letter π.( The symbol π is called 'pi' in English and is pronounced 'pie').
So we can write the formula for the circumference of a circle as: C = πd.
However, the diameter is equal to 2 x radius, (2r), so we can also wrtie this formula as: