Circumference of a circle
When working with circles, we use the word "circumference" to refer to its perimeter. The circumference is just the length all the way around the outside of the circle.
We use π (pi) frequently in circle calculations. This is because π is the link between the circumference and diameter of a circle, and is part of what makes a circle a circle.
To calculate the circumference, we can use:
a) the diameter (measurement from edge to edge through the centre)
In this case, circumference = π x diameter, or C = πD.
b) the radius (measurement from centre to the edge). This is half of the diameter; in other words, two radii make one diameter. So the formula becomes circumference = π x 2 x radius, or C = 2πr.