Circles (pi)

Circle terminology

Here is a list of words associated with circles. Are they familiar to you?

Put your mouse on each word to highlight an example.

These are the abbreviations that we will use:

• C = circumference
• A = area
• d = diameter
• r = radius

Circumference of a circle

The circumference is the length of the edge around a circle.

For any circle, the circumference is:

Or in symbols:

This is true for all circles and so ... is therefore a special, unique number, and we represent it with the Greek letter .

 (The symbol is called 'pi' in English and is pronounced 'pie').

So we can write the formula for the circumference of a circle as:

However, the diameter is equal to , (), so we can also write this formula as:

It does not matter which of these formulae you use. But you must be careful to use the correct length for the formula (the radius or diameter).